# Carburetor information



## gbritnell

The topic of carburetion has come up on several threads, one on Steve's straight eight engine and another one that Chuck started. 
 This is going to be an open discussion on carb operation and design. Everyone who has built an IC engine has used a carb, whether of their own design, a copy of another or a store bought unit. 
 I have been fooling with them for quite awhile and have built several different types.
 When I first built my 302 engine I made up a rather sophisticated carb, by that I mean it had a float, an idle circuit, a high speed circuit and a blade type throttle plate. The first problem was getting the float to seal. The float itself was made from .002 brass shim stock and soldered together. The problem was that the float didn't have enough displacement so it didn't apply much pressure on the needle valve. I eventually eliminated it and went to a gravity feed to the carb. It worked ok but not to my liking.
 I attended the NAMES show at Wyandotte, Michigan the one year and Lee Root had his small Chevy motor operating. It had an oversized Stromberg type carb on it. I tried talking to Lee and asked him questions but he wasn't the most forthcoming person on his accomplishments. Awhile later SIC magazine published the drawings for Lee's carb. I thought, Oh boy, this is just what I need. I built the carb according to the drawings. This was more complicated than the original one I had built. Along with the float and separate fuel circuits it had an accelerator pump and choke. I got it finished and made an adapter plate for my 302. This carb required that the fuel be pressurized to get it from the fuel tank to the carb. Being new to this aspect of the hobby and not being able to gather much information about pumps I made a small piston pump, kind of like on an old Coleman stove. That part of it worked fine but I still had problems with the float sealing. Sometimes it would and sometimes not. 
 I got frustrated fooling with it so I moved on to other carb types. I am including several pictures of the Lee Root carb.
 This evening I will continue with this discussion. My wife and I are going out for our first motorcycle ride of the season.
George


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## cfellows

George, thx for starting this thread. Carburetion seems to be the bane of most model IC engine builders, and it certainly is mine. One thing that constantly puts me off is their seeming complexity. I'm willing to sacrifice some functionality for simplicity. All I really want to do is be able to control the speed of the engine and if it requires some manual tweaking of the mixture when I change speeds, that's OK, too.

I've looked at some drawings, notably from Jerry Howell, but also others, and, not being a draftsman or engineer, I can't make heads or tails of how to build one. So, I'm hoping this thead will evolve into something very worthwhile for all us carburetor challenged folk.

Chuck


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## gbritnell

I guess I should step back and start closer to the beginning. By that I mean I should probably give a little background on carburetion, both full sized and the model type that we're interested in.
 My teenage years were the late 50's and into the 60's. All my buddies were into cars, whether stock or hot rodded. Through my friends and reading the car publications of the day I learned about carburetors, how they worked and how to adjust them.
 The basics are simple, mix the right amount of fuel with the required amount of air and the engine will run properly/cleanly. If it's too rich performance will fall off and the plugs will become sooty and fail. If it's too lean missfiring will occur and heat will build up sometimes to the point of component failure, namely plugs, valves or pistons. 
 The automobile carburetor has four basic stages of operation, starting which requires a richer mixture, idle, acceleration and high speed running. All of these functions are governed by the air flow through the carb. Sometimes the mixture is close to 14.7:1 and sometimes it's not, but generally always favoring the rich side. 
 In the final days of the carburetor's life span they were developed about as far as they could go but were still basically polluters compared to fuel injection so they had to go. 
 The early manufactured carburetors had one adjustment on them, the idle air mixture screws. The rest of the control was by jets and orifices that through engineering was calculated for the best performance. Yes you could change the main jets but without proper measuring equipment it was a seat-of-the-pants affair.
 Motorcycle carburetors have always had much more adjustment to them than automobile carbs. Yes there is still the idle air mixture screw but along with that you could change a needle and jet for mid range operation and the main jet for high rpm performance. Here again without proper measuring tools about the only way you could determine if you had things close was to take the bike out, run it, shut it off and check the spark plugs. Not the most scientific way of doing things but plenty of mechanics got by this way. 
 The most noticeable adjustment was the idle mixture screw. The procedure was to start the engine and let it warm up, adjust the idle stop screw so that the engine would idle as low a possible without stalling and then adjust the mixture screw. First you would turn it in until the engine started to stall (too lean) and then back it out until the engine would slow down (too rich). In the course of doing this as you would near the sweet spot you might have to readjust the stop screw to get the idle speed back down. Once you were close the normal procedure would be to turn the mixture screw about 1/8-1/4 turn to the rich side. This applies to any machine you're working on, car, motorcycle, lawnmower etc., although small engine equipment, lawnmowers, weed eater, snow blowers etc. no longer have adjustment screws on them. The manufacturer calculates what jet sizes the engine needs and builds the carb accordingly, all to meet pollution regulations. 

 Now on to model carburetors. The simplest type would be the fixed venturi type like the ones found on hit and miss engines and for that matter small model airplane engines like the old .049 Cox engines found in control line planes. 
 What is a venturi? The venturi is a reduction in diameter found in the middle of the carburetor body. As the air flows through the carb it's velocity increases in the venturi area and with that a greater vacuum signal is formed. This is where the fuel jet is located. Being as both of these engines are basically fixed speed engines the only thing required is to start them up and adjust the needle until the engine runs at it's best. If you have fooled with a hit&miss engine you will notice that if you adjust the governor screw to bring the rpm up sometimes a small adjustment is needed to get the engine running cleaner. That's because as more air flows through the carb the vacuum signal changes and the fuel must be metered to match that flow.
 Now we'll talk about the variable venturi carb. The early types used a rotating barrel within the body of the carb. If you look at drawings for any of the small IC engine carbs they all used this style, even the model airplane carbs. The principle is the same, the barrel has a smaller diameter hole than the inlet and outlet thus creating a venturi. The problem now is that as the barrel is rotated to allow more or less air through the carb to change the speed of the engine the fuel must be metered to match the airflow. The easiest way to do this is to add what is called an air bleed to the carb. 
 When you start an engine with this type of carb the first thing that you do is open the carb fully and adjust the needle until the maximum rpm is reached. You can hear the engine speed up or slow down as this adjustment is made. As I mentioned earlier once this point is determined the proper thing to do is adjust the needle a little to the rich side just to prevent burning up the engine because of a lean condition. Now as you close the throttle down to idle the engine will become very rich. Why, because you're pulling less air but the needle adjustment hasn't changed. To compensate for this the air bleed is adjusted. The air bleed is nothing more than a hole that goes directly from the outside of the carb into the side of the bore. It has an adjusting screw that opens or closes this hole to lean out the rich condition of the carb. As you adjust this screw you should hear the engine change speed. As the rpm goes up you are leaning the mixture out and getting the engine closer to the proper air/fuel ratio. Now you adjust the stop screw which controls the closed position of the barrel until you can get the engine down as low as it will go without stalling. If you find that the engine doesn't respond to the air bleed adjustment it means that the air bleed hole isn't large enough. 
 Another thing to keep in mind when using a model airplane type carb is that they are designed to use alcohol fuel which has a different air/fuel ratio than gasoline.
 George


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## gbritnell

Here are a couple of pictures of an air bleed type carb. In the first picture you are looking through the venturi of the carb. At the bottom of the bore you can see the jet protruding. At the top center is the actuating arm. The screw at the top right is the throttle stop screw and the screw at the top left is the air bleed adjusting screw.
 The second photo shows the airbleed hole in the side of the carb. I'm sure there is a proper size and placement of this hole but whenever I make a carb of this type I just copy what I have in front of me and make adjustments for the displacement of my engine. Here again this is not a sophisticated model airplane type carb. It's basic function is to idle clean and run clean at high speed. There's not much control over the mid range mixture but then model airplanes don't run at half throttle much. 
George


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## stevehuckss396

Good stuff G!

I made the carb on the Peewee to the shores plans. The air bleed hole was increased a few drill sizes. The motor seemed to respond and it leaned out a bit at idle. I again drilled the air bleed bigger but after a few more sizes, It stopped responding to the hole getting bigger and it still runs rich at idle. 

I have the plans for the Jerry Howell V4 carb he calls the "2 jet" carb. Do you know about this carb or any other carb design that would work well on an engine the size of the peewee. I don't see the point in building another one like the minimix.


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## cobra428

Hi George,
Most of the carbs I have on my airplane engines have the "butterfly" barrel on a helix. It's supposed to give smoother transition and better mid range. As the helix moves in and out the needle valve seat follows along. I'm just going on memory, it's been 5 yrs since playing with them. I'll double check when I get home.

I was even thinking of some venturi size to displacement measurements. Maybe we can interpolate the size for a particulate engine size. I have .10, .60, 1.08 glow 2cyc and 1.20 4cyc glow 4.2ci gas and 85cc gas twin. Multi cyl just need enough carb for one cyl since one cyl will be on intake at a time. My 9 cyl radial calls for a .60ci OS carb. even though it's full disp is ~150cc.

Tony


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## gbritnell

Model airplane carbs have come a long way. These pictures are of an OS carb that has done away with the airbleed. What we have here is a carb that incorporates an idle adjustment, a mid range adjustment and a high speed adjustment. 
The first picture shows the venturi with the metering tube inside of it. The way that it works is it has one tube inside of another. The inner tube has a tapered slot running along the wall of the tube. As the outer tube rotates with the throttle barrel it uncovers more or less of the taper thus controlling the mid range of the engine. The idle has it's own screw adjustment and the high speed is still with the needle valve. The manufacturer gives you approximate presets for the carb but ultimately it is similar to the simple one, adjust for high speed first because the needle controls the fuel flow. Next you do the idle but instead of an air bleed you are moving the postion of the outer tube relative to the inner one which has the tapered slot. On this type of carb you are adjusting the fuel mixture throughout the entire operating range. The last picture shows the mid range and idle adjusting screws. 
 I bought this carb with the idea of using it on my 302 and I sized it accordingly. The only problem was that, as I stated earlier, it's made for controlling methanol and that's not the same a gasoline so even though I calculated for a smaller displacement the fuel ratios are way to high. 
 At this point someone might ask, "why not just use alcohol/methanol?" Gentlemen, I don't need any more complications than I already have. When I get this whole carb thing sorted out maybe I'll try some other type of fuel.
George


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## gbritnell

Last but not least is the latest iteration of the model airplane carb. It's a take-off of the cylindrical slide type motorcycle carb. The slide moves up and down at right angles to the bore. Mounted to it is a tapered needle which comes out of a needle jet. The idle is controlled by how deeply the needle is adjusted into the needle jet. The mid-range is controlled by the taper of the needle. The farther it comes out of the jet the more the fuel flows. And finally when it's wide open the needle controls the amount of fuel. 
 Personally I'm not into RC type activities, Lord knows I spend enough on my end of the hobby, so as far as the merits of each of these carbs you would have to talk to someone that fools with them more than myself. I have been on the RC universe website and read what others have written and as with anything I read some good and some bad for each type of carb. 
George


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## gbritnell

While composing these treatise Steve jumped in with a question about the Jerry Howell carb. Steve, funny you should ask. I was just about to get to that segment. When I said I had built and fooled with many carbs I wasn't kidding. 
 First a little more carb fundamentals background. On an automobile carb as with any carb you have a venturi. We have already covered this but on an auto carb it comes into play twice. Once is the necked down area in the middle of the carb where the high speed nozzle is located. It's more complicated than that but let's just leave it there. 
The next area is at the throttle plate. The throttle plate is the circular device that when rotated opens and closes allowing more or less air into the engine. When it's wide open the main venturi pulls the fuel from the float chamber due to a high vacuum signal but when it's closed, or almost closed, it creates it's own venturi, or high vacuum area. Remember as the air flow goes through a restricted area the velocity increases. With the throttle plate at the idle position there is a small passageway in the wall right next to the throttle plate. This is the idle port. The high vacuum pulls fuel from that port and the amount of fuel is controlled by the idle mixture screw. As the throttle plate starts to open the vacuum signal changes position and if you ever had occasion to look inside an automobile carb you will see a series of holes running up the wall. Each one allows fuel to flow as the throttle is opened until the point that the main venturi takes over. 
 One of the main reasons that modelers don't make this type of carb is because in full sized practice the throttle plate is screwed to the throttle shaft so if you have say a carb with a .280 bore you can imagine how small the screw would be to fix the plate to the shaft. I'm not saying it can't be done, I have one, but it's a pain. 
 Now come Jerry Howell. He makes a little carb that uses a throttle plate but made a different way than it's full sized brothers. He takes a piece of round stock for the throttle shaft and machines the middle of it into a plate, each side having a tapered edge for better air flow. Now instead of drilling tiny internal ports in the body of the carb he drills the shaft and then drills another hole at right angles which exits at the thin edge of the throttle plate. When the plate is at idle position the hole is presented to a high vacuum area and fuel is pulled through it. As the plate opens and the vacuum signal decreases he has another tube which acts like a venturi. It also has a cross drilled hole which supplies fuel. Each circuit has it's own adjusting screw. I built one of his carbs and was trying it out on my Holt. I was having other issues at the time but didn't know exactly what so when I sorted out the other problems I never went back to his carb. The engine did run on it but I didn't give it a fair chance. Here are a couple of pictures of that carb.
George


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## gbritnell

Remember what I said when I started this topic. I would like it to be an open issue about carburetion. Hopefully others who have had good luck with a certain design or type of fuel metering system will wade in and add to the information. 
 Here is some information I have gathered about using a Walbro or similar type carb on our engines. Having noticed that some people were using them I started to make some contacts and find out what was required for their use. 
 Having rebuilt many of these little devils over the years I knew what was inside them but never did know exactly how they worked. I found some very good information online and have saved it on my computer. I sent Steve the PDF file only because it was too large to post here. 
 First off, they are in fact pumper type carbs. The pumping action comes from the pulses created by the crankcase pressure in a 2 cycle engine. What I thought was the pumper part was the diaphragm connected to the small needle valve under the chrome cover on the bottom. This isn't the case. This part is the flow control valve and is operated by the engine vacuum. When there is air flow (vacuum) it acts on the diaphragm an opens the needle to allow fuel to flow. On the top of the carb is another cover which has a plastic or neoprene gasket with little valves cut into it. This is actually the 'pumper' part. 
 Now in talking with Ken Hurst (west coast, supercharged engines) he said that the crankcase pulses aren't necessary, just use a low pressure electric fuel pump. It will flow gas through the little flapper valves and won't flood the engine because the main passage won't be open to the needle valves until there is a vacuum signal from the engine running or turning over. I'm about two days away from finding out. I'm just finishing my gas tank and then I'll let you know. 
 Paul Knapp (Rimfire spark plugs) has a challenger V-8 engine that he uses a Walbro carb on. He made a small air pump to send pulse signals to the carb. 
 Ron Colonna also uses one but he pressurizes his tank with a small plunger before firing up his engine. 
 On a final note, while attending the NAMES show in Wyandotte many years back Lee Root had an inline 4 cylinder engine that ran great. While looking at it I discovered it had a small fuel injector. It was a throttle body type carb like the early automotive types. Lee had built this injector and while the engine was running you could look in the carb and see it pulsing fuel. It was fantastic. The only problem was as I stated earlier, Lee wasn't given to handing out information so a lot of his knowledge and expertise went to the grave with him. I vowed that I would pass along any information and knowledge that I had learned through the years to others in this hobby. Why on earth would I keep it a secret?
George D. Britnell


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## bentprop

George,the slide carb you show is actually nothing new.
I had one on my OS21VF rc racing car engine 30 odd years ago,the carb if i remember right,was made by Picco.The main advantage of this type of carb is the speed of opening,which is important in a race engine.I should also mention that we used .40 sized carbs on .21 size engines.of course,technology has moved on,but I believe this type of carb is still used in rc gas cars today.
I think it's use in home built engines would be fairly limited,because they tend to run a little rich at low speeds.
That said,I'm far from an expert on the subject.


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## mu38&Bg#

There really is no difference in how mixture is metered between the two needle rotary carb and the two needle slide carb. Mid range mixture is controlled by the taper on the needle, but also by the low speed needle setting. They tune OK for gasoline, but end up rich in the mid range. The slide type carb are usually found on RC car engines. These are generally set up for 15-30% nitromethane glow fuel so using such a carb with gasoline will result in a very rich mid range. Airplanes carb seem to work OK, but needles become very sensitive on gasoline. Three needle airplane carbs are few and only found on helicopter engines these days and are also set up for 20-30% nitro. There are some three needle slide carbs for cars.

George, OS in the past has used the flat plate throttle machined in round bar, and YS uses this design currently.


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## black85vette

gbritnell  said:
			
		

> Motorcycle carburetors have always had much more adjustment to them than automobile carbs. Yes there is still the idle air mixture screw but along with that you could change a needle and jet for mid range operation and the main jet for high rpm performance. Here again without proper measuring tools about the only way you could determine if you had things close was to take the bike out, run it, shut it off and check the spark plugs. Not the most scientific way of doing things but plenty of mechanics got by this way.



Yep. Did plenty of the trial and error / seat of the pants tuning way back when.  Had a couple of Ossa dirt bikes with IRZ carbs.  The slide had two needles that had multiple grooves with clips and two jets. So you could adjust the height of each needle to the slide and change the jets for either.  Used to take the bike out to a dry lake bed to tune it so I could run for extended times at any throttle setting, then shut it off quickly and pull the plug. Crude but when it was finally right the engine was awesome and had great response across a wide RPM range, didn't load up / foul, and started every time.

Thanks for the posts George. Lots of good info.  I am still a bit intimidated at the thought of building a carb but am enjoying all the theory and background about them.


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## cobra428

George,
Thar's an old fasinon carb. Get with the times ;D

Tony


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## stevehuckss396

cobra428  said:
			
		

> George,
> Thar's an old fasinon carb. Get with the times ;D



Well, what do you got?


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## Longboy

Thanks George for the model engine carb primer. This is timely for me as I got a new Traxxis carb on the Webster just last week but didn't have the proper procedure for set up. In just grabbing knobs and twisting screws I started with closed throttle and fuel mix screw to get a smooth idle but the engine would starve and stall around 25% throttle opening. I Googled "R/C forums" Tuesday and got a handle as you say to set mixture first at open throttle. The Webster doesn't seem to care for more than 30% open so I'll set up for max rpm at that point for mixture and use the needle under the throttle arm to limit fuel feed for a great idle.   Dave.


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## cfellows

Longboy  said:
			
		

> Thanks George for the model engine carb primer. This is timely for me as I got a new Traxxis carb on the Webster just last week but didn't have the proper procedure for set up. In just grabbing knobs and twisting screws I started with closed throttle and fuel mix screw to get a smooth idle but the engine would starve and stall around 25% throttle opening. I Googled "R/C forums" Tuesday and got a handle as you say to set mixture first at open throttle. The Webster doesn't seem to care for more than 30% open so I'll set up for max rpm at that point for mixture and use the needle under the throttle arm to limit fuel feed for a great idle.  Dave.



Which model did you get and how big is the bore?

Thx...
Chuck


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## Longboy

That's a Traxxis TRX Pro .15 carb from Ebay Chuck. Probaly about a 8-10MM bore.


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## black85vette

George

I am wanting to build the Webster. Don't care about speed control very much. It won't run very much, just for my own entertainment.  Would like it to tune up and start easy. So which carb would be a good choice for a first build (and not lots of experience)?


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## cobra428

stevehuckss396  said:
			
		

> Well, what do you got?



Well Mister Hucks














































Do you want to see more?

I've been in the model airplane hobby for 50 years. I'm a licensed aircraft mechanic and aero engineer and have every pilot license there is

SO WHAT DO YOU HAVE?
Sorry George, don't mean to mess with your thread but me and Mister Hucks seem to have a problem

Tony


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## cfellows

black85vette  said:
			
		

> George
> 
> I am wanting to build the Webster. Don't care about speed control very much. It won't run very much, just for my own entertainment. Would like it to tune up and start easy. So which carb would be a good choice for a first build (and not lots of experience)?



This is kind of what I want to do. Unfortunately, if you don't have some kind of speed control, the only option seems to be to run the engine wide open which isn't a good alternative either. That's why I'm trying to come up with a simple design that allows some kind of speed control, even if I have to tweak the air mixture in between speed adjustments.

Chuck


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## putputman

George, thanks a ton for starting this post. Just a lot of good information on it. I certainly hope this post keeps going as there are a lot more information out there to share.

I can certainly understand why your engines perform so well.


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## gbritnell

Gentlemen, because I have heard so many people ask questions about carburetion I thought I would start this thread. I have presented information and pictures relative to my experiences with carbs. My presentation is not meant to be the end-all of information. I know what it would take to build the ultimate carb for an engine, the problem is without measuring equipment for air flows, vacuum strengths etc. it's just a hit and miss proposition. 
 On another note, I have found that if someone writes something that's misunderstood the best thing is to question the reason for the post. Please don't get into a shoving match to prove a point. 
 BlackVette, the Webster is not a hit and miss type engine so therefore it needs something to control it's rpm. The only thing that will do this is the control of air/fuel into the engine. That being said, I would build a simple air bleed type carb. They are relatively easy to make and can be adjusted to operate fairly well. On Longboy's Webster he uses a factory model airplane carb. In my writings I stated that these carbs are made to flow more fuel than is required by engines running on gasoline. Yes they work but are too sensitive to adjustments. I'm attaching a drawing of the carb I use on my 4 cylinder OHV engine. The bore and stroke of the engine is .75x.875. I'm sure this carb will work on anything from .625-1.00 bore without changing the venturi size. 
George 

View attachment 4 CYL OHV SHT J.pdf


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## kustomkb

Good info!

Thanks for the carb drawing and detailed explanations George.


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## cfellows

Thanks for the drawing, George. I had to study it a bit to understand it. And I do have a couple of questions.

It looks like the idle stop screw is what holds the throttle barrel in the body?

Also, it looks like the air bleed hole is blocked by the throttle body at all positions except at or close to idle?

Finally, do you think this design is scalable down to maybe 3/4 or even half that size?

Thx...
Chuck


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## gbritnell

Hi Chuck,
You're right on all counts. The air bleed hole is open from the closed position to about 1/8-1/4 throttle. It's only needed for the idle mixture. Like I said, the design of these carbs is for idle to wide open. They don't operate at a constant speed in the mid range. By 3/4 do you mean bore size or 3/4 of the drawing size?
George


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## stevehuckss396

cobra428  said:
			
		

> Mister Hucks seem to have a problem



No problem Tony. That was not a personal attack of any kind. You said that georges carbs are old tech. Just wondering what the modern stuff is. All we are doing here is trying to figure out the best route to take for carburation. I was trying to get your input.

My most humble apology!


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## stevehuckss396

gbritnell  said:
			
		

> The air bleed hole is open from the closed position to about 1/8-1/4 throttle.



The shores carb is built and tested with a very small air bleed. Then it is to be opened a drill size at a time and tested again until the engine stops leaning out at full throttle.

When I did that it responded to a point and then the throttle started to go soft. It stopped responding to increases in bleed hole size.

The air bleed hole is at the very bottom of the throttle. The hole starts to get covered as soon as the throttle is twisted. 

How would raising the airbleed hole affect the tuning if it were uncovered longer. I am wondering if the hole is to low and is being covered to quickly. If i raise the hole, a smaller hole would bleed more air for a longer period. It would lean out the idle mixture more.

Is my logic flawed?


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## gbritnell

Hi Steve, no your logic is not flawed. It's one of the comments that I made when I was writing this all out. Not having the ability to measure the exhaust output makes it hard to engineer the carb. The old motorcycle way was to run the engine, shut it off and then read the spark plugs. It doesn't work so well in these sizes. I do know that Rimfire recommends running a richer mixture for the plugs. By that I would assume richer is better than leaner but not rich enough to foul a plug or cause lazy running. 
 The one thing I have found is that the venturi hole can be much smaller than one would think. The next carb I make is going to start out with about a .200 bore and work up from there. When my Holt is in the idle position you can barely see a crack in the thottle bore so this tells me it's providing more than enough air even at these small openings. 
George


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## cfellows

George,

My engine has a bore of .8125" and a stroke of about 1.875 for a total displacement of a tad less than 1 cu. in. While the carb dimensions in your scale will likely work fine, I'm concerned that it will look out of scale on my engine.

I would like to shrink the whole carburetor down by about 40%.  I'm working on a drawing with a venturi bore of 5/32".  Aside from the eyesite and eye/hand coordination of a 66 year old, do you see any problems with that idea?

Thx...
Chuck


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## gbritnell

Hi Chuck, why not make the carb outer dimensions to fit the scale of your engine but just do the internal passages so suit the engine's need. I personally don't like the look of a simple air bleed carb on a small engine so usually when I get the engine to run right I whittle away some of the stock to make it look more attractive. 
 This is what my Holt carb looks like after chopping some of the extra stock off the outside. I even added a dummy float bowl to make it look more the part. These pictures show the governor linkage hooked up also. I would have liked to use the big brass one that I first built but it wouldn't work at all so it was kind of irrelevant. 
George


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## cfellows

Thanks, George. That carb on the Holt really looks nice.

Based on what I'm reading, it sounds like the venturi bore, for all practical purposes isn't very critical. I know my Plumbing engine will run on my little carb with a 1/8" bore venturi. Yet, based on one of your earlier posts, it would just as likely run as good or better on a carb with a 1/4" bore. Is that correct? For a given size engine, is there any benefit of one size over the other?

Thx...
Chuck


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## gbritnell

Hi Chuck,
On a slow running engine the venturi can be made smaller to increase the velocity. The larger the venturi is the less air velocity passing through it. I have read numbers as far as a ratio of engine bore to carb venturi size but have never built one to exactly those numbers. It's a hard thing to assess the proper size for the carb bore. One way you can tell is when the carb is wide open and you can't get the proper adjustment on the needle to make it run clean then the bore should be made smaller. The only problem with this is that who wants to run their engine wide open for 4-5 seconds while they try to figure out the carb settings. It works fine for a model airplane engine because they basically run wide open anyway. 
George


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## stevehuckss396

I over drilled my air bleed hole on the Peewee. I was going to make a new carb but with NAMES comming I was hoping not to have to spend 3 days getting it done.

I over drilled the hole and tapped it 4-40 and made short screw to go into the hole. Then I chucked up the little screw and drilled the small hole thru the screw and screwed the little bugger in. I'm not back to where i was originally but it runs much better and I wont look like a dumb a$$ at names. Well not because of the peewee. Might make another and go smaller on the hole and see what happens. I think I would rather run real rich and idle and rev nice than run like crap.


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## Rustkolector

George,
This is a great thread. I have enjoyed it and have read it a couple times. Some of us like to build slow speed engines that use a throttle type carb (like your Holt), but are best run at constant speed. I have built and purchased a few carbs in my attempts to find the ideal carb and fuel system. What I have found is that a small carb like the OS 2A does well at constant low speed because it has an adjustable air bleed port. Unfortunately, with a .188" throat dia., it is still larger than ideal for a multi-cylinder 1" bore 4 stroke engine. I don't believe they make any RC carbs under .188" with the adjustable air bleed. I built a .100" throat carb for a 1" bore multi-cylinder engine, and it runs well at only 25% open at reasonably low speed. Unfortunately I built it without the benefit of an air bleed for low speed operation, so it is less than an ideal performer on low speed operation. Now I am guessing that a .110" to .125" throat carb with both high and low speed fuel jets would be ideal if it also included an adjustable air bleed port under the throttle for fine tuning. Or better yet, one where the bleed air mixes with the low speed fuel similar to that of larger carbs. Does the Jerry Howell carb come close to this?

With the larger jetting of the few RC carbs I have tried I have noted one practical advantage they offer for 4 stroke model engines. The jets and fuel passages seem to be large enough to allow the easy use of propane as an alternate fuel. It just requires another open turn or two and a demand regulator. It is a very clean burning fuel, but has its own issues as do all fuels. The adjustable air bleed provides the same fine tuning benefits with propane as with gasoline or camp fuel. 


Jeff


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## gbritnell

Hi Jeff, 
 I certainly don't profess to be an expert on the subject of carburetion but I've learned so much in the last few months working on my 302 engine. As has been spelled out in this thread I am currently using an OS 2a also. I have it to the point where the engine runs almost flawlessly, from idle to full acceleration. I have the air bleed wide open and I believe I could open up the diameter of the port a little but I really don't want to alter this carb. I can use it as a test carb for other engines. I ran the engine for almost four hours over the 2 days at NAMES. It was getting quite warm and as it did I would have to richen up the needle one or two clicks to keep it fine tuned but it never faltered. 

 This engine has a bore of 1.00 and a stroke of .900. I can idle it down to 700 rpm and rev it just shy of 5000 rpm. There were fellows stopping by that had Walbros and other types of carbs on their engines and were amazed at the performance of this simple little air bleed carb. As I said and as some others have stated the problems I was encountering were not fuel related but rather ignition problems. 

 When I first built this engine I made a scale copy of an Autolite 2100 carb for it. This carb was pretty much the standard issue for Ford v-8 engines that used a 2 barrel carb. The one I made had a butterfly for the throttle plate with idle passages, a float etc. The biggest problem was getting the float and needle to operate in that small scale. I didn't have much luck. 

 Now that I have the engine running so well I wanted to remake the 2100 carb purely to add to the scale appearance of the engine so I took all of the dimensions from the O.S. carb and sat down at my drawing board and designed a new carb. 
I had to figure out how to get the rotary throttle barrel into it but have the outside look somewhat realistic. I also had to figure out how to get the fuel from the inlet to the needle valve using the scale dimensions from the full sized carb. It took some extra drilling to get the passages into it and I then plugged them with small set screws in case I wanted to alter them in the future. I also wanted to add more detail to the carb to give it that scaled down appearance so I added the accelerator linkage and the automatic choke mechanism. The choke unit sits over top of the needle valve and kind of hides it. I must step back here and say that before I added all of the extra detail I tried the carb out on the engine. It ran almost as well as the OS carb. By that I mean it would still accelerate clean and would idle 'almost' as well as the OS. I had the airbleed wide open and it could use more air at idle just to clean it up a little. I think that the length of the bleed passage added some restriction to the air flow and that is why it was just a tick off of the OS. 

 You can see in the pictures the size of the main venturi bore and the barrel is about .04 smaller in diameter. This gives the barrel hole a diameter of about .160. My needle jet is drilled .036 diameter and I am using a sewing needle soldered into my valve body for mixture control. The diameter of the fuel passage from the fuel inlet to the needle valve is .063. 

 What does all of this mean? I have discovered that most of our carbs are way to big in the venturi department. For an engine having a 1.00 bore I would start out with a carb having a venturi no larger than .200 diameter. You can always go bigger. 

 The first 2 pictures are overall shots of the new carb with all of the window dressing on it. That's a dime sitting next to it. The next 2 pics show the bottom of the carb with a scale next to it to show the venturi size. The last 2 pictures show a closeup of the needle valve area and how I used a plug to fill the bore for the barrel. I keep it in place with a 2-56 countersunk screw. 
You'll have to pardon the dust on the top of the carb as this was the top from the original carb and it's been sitting for quite some time.


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## gbritnell

Here's the last 2 pictures.
George


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## gld

One issue that has not been covered---atmospheric conditions. Namely humidity. My Snow engine ran perfect at Rapid City last fall and NAMES this spring. Last week I tried to run it and it will not keep runing..Will only run about 4 or 5 seconds and die as if the fuel was shut off. Always starts right back with the same results. My carb is off a Thunder Tiger .15... Only difference is we have very high humidity here now.


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## gbritnell

Hi Garry,
You're right about atmospheric conditions but I think they're more applicable to the ignition or should I say combustion chamber rather than the fuel alone. If you are using gasoline it doesn't have that much of an attraction to water but if you are using some type of alcohol blend it can draw moisture with it. 
 What I have found when the humidity is high is that more condensation forms in the combustion chamber and therefore tends to short the spark out. I have several hit and miss engines that this condition occurs. Usually if I take the spark plug out, wipe or blow it off and put it back in, the engine will start instantly but within a few seconds will stop again. Once there is some heat built up in the plug, head and cylinder the situation seems to go away. 
gbritnell


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## Lakc

Atmospheric pressure and temperature are the primary inputs in modern electronic fuel injection. Since all we are doing is completing a chemical reaction by burning the fuel to make the power, it is essential to know how much oxygen you have to work with. 

 Carburetors, even full size ones, have a tough job to do. With gasoline, it has to keep a linear flow of approximately 14.7 pounds of air to every pound of fuel, under full power increase that to approx 9 pounds of air for every pound of fuel, and starting alone can be as rich as 3:1. 

 Forgive me if I steal some math from the web here, but apparently, a pound of air is approx 2894 gallons at standard temp and pressure, and a pound of gasoline is roughly 1/6 gallon. In the best case, that's a whole lot of air for a tiny amount of fuel. When you talk starting or max power output mixtures, you have much less air to work with. If you want to run on alcohol fuel, you basically just double the gasoline numbers on the fuel side. 

 I will leave the heavy math to those on this forum who can do it so much more eloquently (and correctly) then I. Anyway you look at it, our small engines move only tiny amounts of air. That does not leave us with much leeway at all to move and vaporize minuscule amounts of fuel.

 That said, as much as I love carburetors, and do have a few ideas of my own to try out, I think they are the wrong solution to our problem at this scale. Positive fuel displacement, like mechanical fuel injection, or just plain pressurized gaseous fuels (propane or butane) make a lot more sense in some areas. 

 While any number of carburetor designs can fulfill the needs to make our beautiful music on the bench at a show, my interests were always geared toward what kind of work we can perform with our engines. That will require a robust fuel system.


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## T70MkIII

I would love to see some scale fuel injection ideas.


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## gbritnell

Quite a few years ago I had attended the NAMES show in Wyandotte, Michigan. There was a fellow there by the name of Lee Root. He always accompanied Robert Washburn, the editor of SIC. 
 The one year he had his 4 cylinder DOHC engine running and it had a small throttle body fuel injection system. It worked great and you could look inside the throttle bore of the carb and see it spraying gas while it was running.
 Sadly Mr. Root wasn't one to write down or share his information and this marvelous piece of engineering is only available for display. 
 It is currently on loan to the Sherline Craftsmanship Museum through the courtesy of Mr. Paul Knapp and can be seen here:
http://www.craftsmanshipmuseum.com/images/k104-RootDOHC2.jpg
George Britnell


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## Lakc

gbritnell  said:
			
		

> It is currently on loan to the Sherline Craftsmanship Museum through the courtesy of Mr. Paul Knapp and can be seen here:
> http://www.craftsmanshipmuseum.com/images/k104-RootDOHC2.jpg



 I hate that site, they keep updating it and I loose an hour+ every time I go there. 

 If you spend any time on Kinsler or Hillborn mechanical fuel injection systems, you find they are a simple positive displacement pump with a whole bunch of calibrated leaks. I like to think even I can make things that leak, but the last pump I tried wouldnt even turn. :'( I will get there someday.... At the end of all these leaks, you flow the fuel past a barrel valve to individual injectors, which are very simple venturi's. 

 Here is a pic I took from way back in my racing mechanic days. The pump is a rotary vane type and belt driven right off the crankshaft at approximately bottom center. The topmost tank is the fuel tank (alcohol), the smaller tank is the radiator. You can see the huge (-6?) line feeding the pump is running directly underneath, and is the lowest fitting. The T right above that goes off to a brass body that contains a combination jet/pressure relief setup (calibrated leak #1) and dumps into the other T back into the tank. The other end of the T goes through the fuel filter next to the distributor and into the manual fuel shutoff valve which has its own return to the tank. The outlet of the shutoff valve goes to the barrel valve which is partially obscured by the #3 cylinder injector stack. The barrel valve is a variable leak with the return heading back to the tank on the rightmost fitting in the picture. What doesnt leak by the barrel valve flows to the 8 black hoses to the brass injectors underneath each throttle plate. The fitting at the very back of the pump runs to the "can" just forward of the fuel filter. That is a jet selector that allows you to choose leak #3 in 5 different sizes if I recall. It exits and enters the tank in the engine side of the T. Sorry if that all sounds about as simple as the tax code, but there it is.


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## T70MkIII

Thanks for the link, George - I missed that one on my last look at the site.

Thanks too, Lakc - I like the concept. I'll have to ponder this some more. Plenty of leaks around here now the rainy season has set in, but they're all too large for a 1/4 scale V8. None of them taste of alcohol either...


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## Joachim Steinke

Hallo everyone,

as fabulous George has started this very informative article about carburators some weeks ago I like to show you the new carb version I have designed a month ago for my latest 0.25 cubic inch 4-cycle gasoline driven engine.

My first version was a simple carb like the radio control people use on there methanol engines. I was inpatient and wanted to get the engine running, so I wasnt in the mood for long experiments and complex extra craftsmanship on this marginal component, but this was far wrong.ha ha ha.








The main dimensions are 6mm (0.24in) of free air passage and a reduced venturi area of 4.8mm bore (0.19in) in the rotating barrel type throttle.














I didnt last long to notice, that this arrangement was not the first choice for my need. Okay, the engine started from time to time, but it doesnt want to run longer than 20 seconds or so and it showed a lousy idle behaviour. Additional the gasoline level had a prominent influence on the whole function, being to low starting was nearly impossible, hanging the reservoir too high means the permanent danger of over floating the intake area.

Okay, to get at least any results with the existing material I tried to modify the venturi area. I made the fuel jet cylinder larger in diameter (from 2.4 up to 3mm) and let it now reach completely into the other side of the throttle barrel which means a quite heavy reduction of free section.








As George already mentioned, we tend to oversize the intake dimensions of our little gasoline motors, especially when we refer to comparable methanol engines. So, with this reduced width my engine was running a lot better. Suddenly I was able to speed up over 7000 rpm, could make long runs of 10 minutes ore more and the idle control was bearable too.

What left was a quite lousy starting behaviour, sometimes immediately running and then on other times it could drive me crazy, and the old problems with the influence of gasoline level.

My idea was that especially for the starting procedure the fuel jet needs the right fuel pressure. As the intake volume has only a small amount the vacuum from the venturi is too weak for the right starting mixture. It seems, that venturi proportions which are suitable for full throttle run dont work good for starting the engine at nearly idle setting. There is simply too little sucking on the fuel passages.

Okay, about three months after finishing the main engine I designed a complete new carburator.ha ha ha. 

The new one is a mixture of a simple motorcycle type and a two needle methanol motor carb. It has a sliding barrel type throttle which contains the linear travelling idle needle and it got a float chamber too.








It took me some time to decide about the free section dimension. First of all, the intake ways of my engine should remain as they are. Second, I would like to use the carb on my next motor which will get a slightly larger displacement (at least for the first testing). So I made a 6mm (0.24in) main bore again with a venturi restriction to 5mm (0.2in). My thought was, that with a sliding barrel, the shape of the gas passage could still work well, even if the throttle is only half way (or three quarter) open at full speed. 

Here comes the inner life of my construction:








From the float needle valve the fuel flows through the float chamber and reaches the main body from underneath into an upright channel which contains the main mixture needle on its top. This needle has a 10deg cone and is driven by a fine pitch screw for a sensitive regulation. 

From the main needle it goes over to the backside of the fuel jet tube. This little, single tube is held by a plug screw and easy exchangeable for further experiments with its functional proportions. All parts are sealed with o-rings, only a little bit more effort while building it, but no more problems with fuel and (more important because hard to detect) air leakage afterwards.








The remaining part is the sliding barrel type throttle valve, it goes with 6mm (0.24in) diameter through the 5mm (0.2in) venturi bore. The idle and half speed needle is also easy exchangeable, it is obvious that this needle might need a lot of experimenting to find the right shape and dimension later on.








End of part one


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## Joachim Steinke

Part two

Okay, now its time to have a look at the real parts. They are made of high strength aluminium (7075) and brass, the valve needles are made of 1.5mm (0.06in) carbon steel.













The aluminium float is manufactured with a wall thickness of 0.01in first and after glueing both parts together with epoxy I reduced the material down to 0.003in by turning it from the outside. 








And finally assembled it looked like that:














Next came the testing of the float. Its dipping to 85%, which will be okay for its duty, but you can see why I had to reduce the weight so extreme. With such a small displacement (resp. a small float diameter) the ascending force is very small too. To enhance the closing force a little bit I actuate the valve by a small lever arm.








Now I could fix the fuel level. I have made an extra hose connector at the underside of the float chamber, so I can easy read the true level in the chamber. I then set the level about 0.04in under the fuel jet centre by building a suitable distance tube for the main body connection. The hose connector is also very useful for venting the float chamber when filling the whole system with fuel.








At last I had to think about the right shape of the idle needle. Here is a view inside the venturi at all four throttle positions








and a section through the complete system.







As the barrel travel from idle to half speed is only about 0.03in the needle has to be very slender. 








To get an idea of a suitable shape and cone angle I first calculated the cross section of the main fuel valve. I have an opening of this valve (for a good full speed) up to 300deg from totally closed. Knowing the main needle cone and the screw pitch I could get an approximate value of the main valve aperture.

The aim for my first tryout was to use a cone which reduces the via main valve available fuel flow within the barrel travel from idle to half speed in the range of 25%. That leads to a needle cone with 1.5deg and a max. diameter of 0.03in. The principle is shown here, sorry for the German inscription, but I think you can get an idea of the whole thing anyway.








The grinding of such small and slender needles is always a bit fiddly. Fortunately I had build a small universal tool and cutter grinder last year at which I can install a second driven spindle. With this equipment a precise needle grinding is easy to do. But otherwise this can be done with a Dremel on the lathe as well.








Okay, now it was time for the final commissioning. It took a lot of patience to get the engine running with this completely new system, too many parameters to maintainha ha ha

But finally I managed to get a perfect full speed run (with still fully retracted idle needle), after that it was quite easy to find a good position for the needle to take the desired effect of leaning the mixture more and more towards idle position. In addition and of particular importance the starting problems are vanished. For the cold engine I have to choke (finger closes the intake) one turn and can start at half throttle with a couple of turns then. And with hot motor it will do some turns at idle position before switching to half throttle.

I dont know if this carb is the ultimo ratio.ha ha ha., but Im a good way satisfied with the result. 

Okay, after a really long and boring post Im closing with two videos showing the new carb operating on the motor

http://pl-hi.de/JST/SLVE/SLVE_RUNNING_CARB_II_01.mpg

http://pl-hi.de/JST/SLVE/SLVE_RUNNING_CARB_II_02.mpg

and two pictures of the now nearly completed engine.














Achim


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## gbritnell

Hi Joachim,
That is a piece of jewelry if I ever saw one. What a great job you did on that carburetor. I like your idea of making the float. I have tried several different ways of building them but never had much success, not the building part but getting enough force to close the needle properly. 
 I tried a small RC truck carb which is somewhat like a motorcycle carb in that it has a sliding barrel to change the venturi area and it has a needle attached which pulls out of a needle jet. I think the problem with it was that the needle and jet weren't the right proportions for proper fuel metering. 
 Isn't it great when you get these little engines running like they should? 
 Maybe you could make up a drawing for your carb for those who would like to try building one. I for one would certainly like to try. 
Thanks again for your comprehensive building information.
George


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## Lakc

Simply beautiful, or perhaps, a better word would be henreißen?

Most commercial carburetors are built with three main circuits, idle, midrange, and cruise. You seem to have done quite well here emulating this.


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## Gemhund

Absolutely, utterly beautiful workmanship. Far better than any jewellery!


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## Joachim Steinke

Hallo George,

first of all, thanks a lot for your appreciation. I observe your building logs and other publications related to IC engines for a long time now and consider you as a real distinguished model maker and engine builder. 

Especially your new 90deg v-twin engine fascinated me so much that I was always impatient waiting for the next continuation of the reportha ha ha.. Beside a straight and clear functional inner design it is your detailed and harmonious shape forming of the housings (without using any castings and CNC either!) that create the particular character and excellent appearance of your model engines. 

And YES, you are right, its worth all the work and technical difficulties when the little engine is running at last. To my opinion, nothing else can create such a fascination for a self developed piece of craftsmanship. And to the steam engine people, please dont take that too seriously.ha ha ha! I have also designed and build a small steam engine with real Stevenson gearing three years ago and had much fun to do this, but finally I found out IC engines are more fascinating for me..everybody has to be different.. 

To your problems with the needle seal of the float chamber: 

I had the same concern owing to the closing force as you, but maybe I was only on the lucky side with my construction. First I wanted to make a long needle travelling through the whole floats centre and let the float be operated by this needle guidance, much like some simple bike carbs from the 50ties are designed. But this bike carbs are a lot bigger and I was in concern if the ascending force of the miniature float will be sufficient enough. In addition I remembered problems with such types of float chamber in my youth when repairing my own light motorbikes. If there is any misaligning or minor trapping in the needle guiding the valve tend to leak, especially under heavy motor vibrations.

So I decided to build this little tin lever arm which supports the float nearly without any unwanted interaction. But the efficient relationship of this lever is really small, perhaps 1:1.5 or 1:1.7. Depending on the drifting ascending centre and the also rotating mass of the swinging float (and its, in relationship, heavy mounting components) the exact value of the lateral point is not easy to define. Anyway, it was impossible to fit much more lever effect in this chamber dimension. The only way to do this is to build an integrated float chamber directly beneath the fuel jet and intake area like all modern bike carbs have. Then you can make a doughnut shaped float of greater diameter which gives you two advantages, more displacement and a lot more lever relation possibility. In addition, depending on the small distance of the lateral point to centre of the fuel jet, you will get a float which is working widely independent of the carbs position in 3D space. But as I dont want to power up a mini motorbike with it this aspect might be negligible. 

The needle seat might be a critical point too. In such constructions the only support for opening the valve comes from gravity (0.1g, dont cough if holding it on the palm of your handit might be gone forever.ha ha ha) and the little fluid pressure on the very small needle cross section. I made the valve seat in the brass connecting screw with a 60deg centre bore. Because I wanted only a small area of contact between the two parts the needle cone was then turned with a light undercut of 59deg. 

My pressure lubrication system (a little gear pump) makes some trouble transporting too much oil via the sleeve valve circumference into the cylinder ports. So it is disabled and I run the engine with a 1:40 gasoline/oil mixture now. With the oil mixture I had some doubt if the needle valve will tend to adhere in the cone, but it works quite well. I think the vibrations of the engine will do a good job too, they generate some breakaway force which, in addition supports the needle movement and help the centring of the needle cone. 

To your legitimate suggestion about publishing a complete technical drawing of this carburator:

I have not yet decided if I want to do this. First of all, for my own production needs I am used to work direct with my 3D CAD model. So normally I dont have automatically generated 2D plans which everyone can easily understand and which can be used as complete working plans. This has do be done and would afford some time to do it correctly. 

On the other hand I believe my posts contain all the substantial information one should need. Unlike to some other people I dont make a mystery out of whats going on inside the little black box. The included plans show all construction principles in detail and mention all important functional dimensions. So nearly everyone should be able to design his own version of the shown component. By the way, I never wanted to build engines with other peoples plan, but that is my own personal attitude. 

To my opinion the 1 to 1 copy of a single accessory like a carb is normally not very useful as the variety of your own environment will determine too many changes. For instance, if you have a differing displacement the venturi might need other dimensions followed by a different fuel jet design and last but not least a different idle needle shape. 

I hope you can understand my attitude a bit.

Achim


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## kuhncw

Hello Achim,

You said in your post, this was a 4 cycle engine. Would you please tell us a bit about where the inlet and exhaust valves are located and how they are operated. 

Your workmanship is just beautiful on this engine and carb.

Regards,

Chuck Kuhn


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## cidrontmg

Hello Achim,
A bit off-topic, I know  :hDe: but in the first video, there´s a neat propeller the motor is driving. I don´t see any maker´s name, did you also make that yourself?
It seems to be a wooden "pusher" prop, as the Americans prefer to call it, or "Linksläufer". The motor seems to rotate clockwise (seen from the front). Most small ( and not only small...) aircraft motors run counterclockwise, so to get proper cooling to the motor, the prop must be a lefty, of course.
I´m also looking for a "left handed" wood propeller, about the same size as yours, but have been rather unlucky so far. So in case you did not make the prop yourself, where did you get it?
Your website and posts have given me hours and hours of pleasure, admiring the amazing things you have made. Respect!


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## Joachim Steinke

Hallo Cidrontmg, 

you are right, unusual to normal RC model motors the shown engine is running clockwise.

I couldnt find a wooden Linksläufer with this dimension (10, with approximately 7 pitch) on the market as well. And as I must have excess to the central collet nut (with a 10mm socket wrench), I need a relative large diameter of the midsection too, which is totally unusual for propellers of this dimension.

So I made two wooden propellers myself, a simple one from a beech wood piece and a fancy one from precious wood. For that I glued 6 layers of pear and maple veneer (each of 2mm thickness) together, the rest is pure manual work with different rasps and some sanding paper, a nice job while watching the usual boring TV.h ha ha..

@Chuck: This engine has no poppet valves, the intake and exhaust is operated by a single sleeve that performs a rotary and shifting oscillation around the piston. This, in the English language area so called sleeve valve timing, is similar to, for example, the system Bristol Motor Company used in there famous Hercules and Aquila radial engines 70 years ago. 

In this drawing you can see the dark grey sleeve which is located between piston and main cylinder. The movement comes from the little ball link, driven by the timing shaft gear which is similar to a cam shaft in usual engines.






The best way to get an idea of the working mechanism is to have a look at this animation which I have made while thinking about my design:

http://pl-hi.de/JST/SLVE/SLVE_PORTS_02.wmv

With this information it might be easier to understand the function of the single parts shown here:








And this is the timing diagram and the port location in a flat projection of the cylinders circumference:








what leads to a port cross section loop over crank degrees looking like this:








But I dont want to bother this topic about carburators too much with more details about my sleeve valve engine. You can have a look at a complete report on my web site, sorry, the text is in German only 

http://www.metallmodellbau.de/

or for instance on Mad Modder Forum, where I also published a full length building report in the building logs area.

Achim


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## kuhncw

Achim, thank you for the photos, 3D models, and link to your website. I had not considered the engine might have a sleeve valve. 

Regards,

Chuck Kuhn


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## Admiral_dk

Hello Joachim

I'm very impressed with your "vergasser" / carburator construction, it's a lot like the old Del'Orto that we had in my youth for 50-250cc motorbikes. They wheren't easy to optimize, because of the horizontal orientation of the mainjet, needle and throttle valve. The good part was that they worked in almost all orientations of the venturi.

I wouldn't be surprised if you can improve the design a bit by turning the throttlevalve to vertical and experiment with the exact fuel level - how many mm. below the venturi. Modern motorcycle carburators have a "pre-mix" function in the needle jet, where air from the outside is drawn into the jet and mixet with the fuel from below and sucked up into the venturi and I wonder if this could be done in the smal scale and stil work as an improvement too.

Then there's direct injection and I got drawings on how it's done in model size and it works - unfortunately in a book and therefore under copyright ....
The princip is simple : A leaky gear pump is used, as the revs rise, the pump get's less and less leaky because of the fuels resistance to movement. This increase the preasure and amount supplied to the engine in an almost perfect relation to how much is needed. The size of the gear and the leak (distance between gears) is the important measurement. It's important to have return valves in the system too, in order to prevent air in the system - he used simple ball and spring valves. I'm sorry to say that he do NOT publish any formula for calculating the gears.


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## gbritnell

One of the problems in designing a carburetor for these small engines is that most of the engines that are scratch built are unique. By that I mean each one could have different valves sizes, port diameters, port lengths and type of cooling would also have an effect. A carburetor that will be fine tuned to run on one engine might not work so well on another. 
 With hit & miss engines the rpm is somewhat constant so the design of the carb can be quite simple, a venturi and a fuel control needle and port. As we have discussed previously, generally the smaller the venturi is the higher the air velocity and therefore the better fuel atomization. 
 The next step up would be engines that run at 2 speeds, idle and high speed, much like a utility engine that would be found in a lawnmower or some similar implement. The old Briggs type engines had very rudimentary carbs, no float, just a dip tube that went into the tank from which the fuel was drawn into the idle port during high vacuum and into the main venturi during high speed operation. 
 What we are asking our little engines to do is run well, and clean, throughout the rpm operating range. This calls for a more sophisticated type of carb, much like Achim has presented. In most cases it's not the machining of the individual parts that causes the problems but rather having no way of measuring how rich or lean the mixture is at any given rpm. 
 If anyone has ever 'tuned' a carb the procedure is to warm the engine, run the rpm up and adjust for the highest, or cleanest rpm. Once that point is achieved the needle is then turned around a quarter of a turn to the rich side. The reason being that if you leave it where it runs the best it's on the verge of being lean and you can harm the engine. Next you bring the engine speed down to a point where it will run as slow as possible without stalling and do the same as you did for the high speed adjustment. This works fine for an engine with a 2.00 inch or larger bore because if you're a little on the rich side the engine can tolerate it but when working with our small engines the fuel control has to be so precise because they won't tolerate either a rich or lean condition for very long. 
 As for using a fuel injection system of the type that Admiral_dk describes, yes it will work but as I started out saying each engine is unique and has specific fuel requirements. Now add in the equations of how loose to make the gears and what size jetting to use just adds to the complexity. And for the statement that as the rpm increases the fuel is provided at just the perfect relationship, this might be so but on an engine with what specifications. 
 I haven't read of or spoken with anyone that has come up with the definitive solution for model engine carburetion. It all takes a basic carb design and then a great many hours of tinkering. 
George


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## Lakc

gbritnell  said:
			
		

> In most cases it's not the machining of the individual parts that causes the problems but rather having no way of measuring how rich or lean the mixture is at any given rpm.



 Thats an easy fix from back in the early days of US auto emission controls. Chrysler used to publish and subscribe to the "propane enriched idle" method, where the idle mixture was set at a specified rpm, and expected to increase X amount of rpm when a metered amount of propane was introduced to find the "best rich" setting. 

 For example, an engine idle rpm was set at 650, with a propane enriched figure given of 725. You would set the idle mixture at best rich and the idle rpm to 725, then lean the mixture out until 650 rpm was reached. Adding propane down the carburetor in the correct amount would jump the idle speed back to 725 verifying you had legally set the idle mixture. 

 Propane will replace any fuel in an IC engine at any rpm. If you think your lean, add a little propane and if the rpm goes up, you are, if the rpm goes down, your perfect or rich already. The trick is only in having a fine enough adjustment to meter the propane with. Butane cigarette lighters might even be more appropriate for small engines. 

 The really tricky part is the nearly logarithmic scale that fuel needs to be added with LOAD at a given rpm. The only load we have on a display engine is the crankshaft, flywheel, and pistons themselves. Bolt on a propeller or some other load and making an engine do some work will take a variable load like a dynamometer along with propane to straighten out the carburation.


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## mu38&Bg#

I think the issue with engine that are run with no load is low intake tract velocity. Surely a carb that is sized to give a reasonable RPM at WOT gives plenty of venturi velocity. But, velocity in the runners is low and there is probably a significant amount of fuel condensing on the walls. This delays the response of the engine to needle setting substantially. The problem is much worse when the engine is cold and requires enrichment to start and run. A separate fuel circuit that could be turned on when needed for enrichment might be a good idea. Walbro carbs are fairly simple. The fuel demand regulator (the diaphragm) replaces a float. Otherwise it's just a 2 circuit carb, but one that has been engineered for the flow conditions encountered in the engine it's on.

In a model engine the correct idle rich setting is one that allows a good transition to high throttle without stalling. These carbs have no acceleration pump, so fuel must already be in the intake manifold due to rich idle mixture when you crack the throttle open. It takes a certain amount of time for this fuel to accumulate and dissipate at various operating conditions, so it's always a compromise. Chasing needle setting without understanding this could lead to a headache. Most model engines are run very rich. Ideal mixture for model engines loaded or not is performance based, not emissions related. The mixtures are set so the engine performs as expected, rather than to meet a certain air-fuel ratio or other specification.

You could run it as lean as you want it's not going to detonate unless maybe you're fueling it with kerosene. You can't run it too rich because the low flow will load up the intake.

The biggest problem I've found when burning kerosene or gasoline is the change in required mixture for a cold engine. If you tune a simple carb for a hot engine it will run poorly until the engine is up to operating temperature. This can take quite a long time. I have an OS FS-52 glow four stroke converted to diesel. Loaded up with a big prop running about 4000 RPM at wide open throttle, it takes well over a minute to reach full temperature. The mixture requirement constantly changes during warm-up, but setting the idle rich will usually tolerate the cold to hot range.

Admiral, do you by chance have a title for the book with the fuel injection details?


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## Admiral_dk

Hi DieselPilot

Yes I've got it at home and it's in German .... It's about building multicylinder model engines, as in-line, Vee and Boxer. I've just Googled :

BUCHMTB33  
MTB33-modell-technik-berater 
Boxer, Reihen- und V-Motoren als Modell 
Autor: Hubert W. Schillings

http://www.rc-machines.com/index.php?navi_1=product&navi_product_1=search&navi_product_2=techn&start=3

It's only a part of the carburation chapter, but you can see one of his injected models on the cover in the above link.

But perhaps you should consider getting the numbers if SIC where there's a build of a 10cc true diesel engine, including injector, pump and the works, if you're more into Diesels.


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## mu38&Bg#

Thanks! I'll look that book up. I can deal with the German, just takes a while. I've seen photos of the Schillings engines. I didn't realize he was making mechinjection systems for those.

I'm also aware of The Dux.


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## Admiral_dk

PLEASE do NOT pay too much for that book .....

He gives a lot of hints, but not much in real info - especially in regard to fuel injection - I'm not sure I would have bought it if I've known how little "hard evidence" there's in that book. It's not a complete waste of money - he do give you a lot of ideas on how-to make crankshafts and certain other bits and pieces.

I'll try to give you an idea and the injection system that he and his son "invented". They claim about 20% power increase compared to carburetors on the same engines and easier starting.

Fueltank -> one way valve -> + return -> leaky fuelpump -> return valve (to fine adjust fuel amount) & -> one way valve -> to all injectors (each containing a one way valve and a adjustable jet) & a (probably adjustable) one way return valve -> fueltank. In other words, the fuel pump is short-circuited by the first ajustabled jet. The individual injectors are adjustable (and placed close to the air-entry into the engine) and I suspect that the last valve is connected to the accelerator, but I'm not sure - the drawing isn't clear, it just states that it's shown in "Leerlauf" ~ idle. There's about 100-150 words on how it works, and that's that  ???


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## mu38&Bg#

Oh, You can get more info from the YS model engine patent.

http://www.google.com/patents?id=zcYMAAAAEBAJ&zoom=4&pg=PA1#v=onepage&q&f=false


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## Admiral_dk

Thank you DieselPilot for that link - very informative, but also with (intentional ?) errors. :

I'm not sure this is a good system - it appears that it suplies the same amount of fuel no matter how much throttle is applied. Then there's the issue of isolating the fuel circuit from the lubrication circuit at the pump - it can be done, but it requires some very small o-rings. That's not to say that the camshaft (perhaps with an extra cam) couldn't be used as an intricate part of the pump with a perfect timing and the same could be said if you consider a diesel system.


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## mu38&Bg#

The only items not shown are the high speed needle and idle air bleed needle. The system has been used as described in their production engines for over 10 years. It works very well. The pressure regulator is referenced to manifold pressure, because the engine uses the crankcase to supercharge the intake at up to 9 PSI. In this type of engine there is no isolated lubrication circuit. The fuel carries the oil, total loss system.


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## Admiral_dk

You're right - I found a complete service manual on the DS 170 (never seen or heard about them before) .... but it do include the seals in form of o-rings.

I must admit that the regulator part confused me - I got it the wrong way around - it doesn't regulate at max revs, but lowers the amount of fuel injected as revs goes down. That and the "missing mainjet" confused me - now at makes sense.

As for using the crankcase two-stroke style to boost the input on a four-stroke - that's not new to me, it's been done to full size racing motorcycles in the eighties - just haven't seen on a model before.


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## gbritnell

When we first got on the discussion about fuel injection for model engines I had made the comment that anything is possible but given the size of our engines and therefore the size of the orifices and jetting the experimentation becomes almost impossible. 

 When I was growing up and fooled with race cars we had a Hilborn fuel injection unit on one of our engines. Being that the engine basically idled or ran wide open the injection system was quite simple. The injectors fed fuel to the cylinders through a jet which was changeable. To feed 8 cylinders and not overfeed or underfeed them you put another metering jet (pill) in the return line. This would maintain a somewhat regulated amount of fuel to the cylinders. Remember this was a race engine and only needed to run at one speed. 

 What has been said about this fellow's injection system reminds me of the Hilborn system, tweak this, adjust that, read the spark plugs and then start over. The only difference is that at least on a full sized engine you could 'read' the plugs. 

 Apparently there are no followers or contributors to this forum that are knowledgeable in miniature fuel injection systems. That's not to say that there isn't someone out there that is, it's just they haven't helped us along with this topic. 

 Not many years back we were all using automotive points, twelve volt batteries and coils and home made spark plugs to spark our engines. Now we have several different suppliers of electronic ignitions, spark plugs and the associated components. Who knows, in a couple of years maybe someone who has access to a lazer for making minute holes will come up with a miniature injector for our engines. Wait a minute, this will then require throttle position sensors and rpm readers and temperature sensors to adjust the fuel metering under varying conditions. Will it ever end! 

 I will be the first to admit that when I bolted the OS 2a carb on my 302 and fired it up I was amazed, astonished, even flabbergasted at how it ran. What type of fuel to air ration it has at any given rpm I haven't a clue, but I'm extremely happy with it. I have been keeping track and I now have over 7 hours of running time on my engine and it still starts and runs great so for the time being I will continue to use a variation of this type of carb on my future builds. 

 I would love to have some type of EFI on one of my engines but I'm afraid that's way beyond my resources. 
George


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## Lakc

gbritnell  said:
			
		

> Apparently there are no followers or contributors to this forum that are knowledgeable in miniature fuel injection systems. That's not to say that there isn't someone out there that is, it's just they haven't helped us along with this topic.



 I have had that engine and fuel system three quarters done for the last 12 years. I have since sucessfully overcome many skill, material, and design challenges, and while its been a labor of love, there has been much frustration along the way. Last year, I managed to convert the existing napkin and graph paper sketches to CAD, but I had to change laptops and can no longer run cad off my thumb drive. It was perhaps an overly ambitious first project. (understatement of the year) The biggest challenge, one which I am failing to overcome, is finding time. Between home, work, family, etc., all my spare time seems to evaporate. 
I have the fifth or sixth iteration of the cylinder heads ready to build, and have already played with the injection pump design and block cavity. Once I get finished cylinder heads that I am happy with I can cast another block (would be the seventh or eighth one of those) and then get to the FUN part of the job, the fuel injection.


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## stevehuckss396

Lakc  said:
			
		

> I have had that engine and fuel system three quarters done for the last 12 years. I have since sucessfully overcome many skill, material, and design challenges, and while its been a labor of love, there has been much frustration along the way. Last year, I managed to convert the existing napkin and graph paper sketches to CAD, but I had to change laptops and can no longer run cad off my thumb drive. It was perhaps an overly ambitious first project.



I see you are in the Detroit area. Maybe I could interest you in combining our spare time and work together to get the injection project back on track. I have good cad skills and machines to make prototype parts. I would love to inject one of my V8's

PM if interested!

Until then i will stick to my original plan with the drum style carb. This one will have to be adjusted with a small screw driver. There will be a 4-40 screw in the front that will be drilled to allow the air bleed. I will be able to swap out different screws (jets) with different hole sizes. If I over shoot the bleed size I will be able to go back unlike the carbs where you drill the carb body.


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## Lakc

Nice looking carb there Steve! Thats not too far off of real life practice, as a lot of Japaneese carburator designs use replaceable screw in air bleeds. Holly uses pressed in brass bleeds, we would measure and swap them on the dynomometer with a kit of gas orifice plug gauges. 

Continious flow fuel injection is simpler then a full-size carburator, but very similar to the model carburators we use. In fact, its essentially identical to adding a Perry oscillating fuel pump, to a barrel carb that moves laterally pulling the needle further from the seat.
http://www.perrypumps.com/prod02.htm

It gets a little trickier when you want to produce maximum power into a variable load at different throttle openings. With the possible exception of the Chevrolet Rochester fuel injection of the late 50's, most of the remaining fuel injection systems are designed for drag racing, not driveability. The trickiest part to build is either the pump, or what they refer to as a barrel valve spool. On the Hillborn systems I worked on, that was a V shaped depression machined onto the outside of a rotating valve, which linked directly to the throttle. I couldnt find a picture of that exact valve, but there is a slotted type barrel valve spool, along with some other information at this site.
http://www.kenlowe.com.au/LFS_injector_hardware.htm


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## mu38&Bg#

The only system I've read anything about is the Bosch Jetronic. Bosch calls all of their fuel injection Jetronic. There are many versions, the earliest open loop mechanical. There is some good information about the system on the net. There is even manual available for download.

http://www.jc42.dial.pipex.com/alpina/Injection/kugelfischer_injection_Description.htm

http://www.bmw2002faq.com/content/view/66/32/#section1

http://www.scribd.com/doc/3299223/Bosch-KJetronic-Fuel-Injection-Manual-


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