# Model I.C. Carb with Throttle



## Brian Rupnow (Mar 11, 2014)

Myself and another fellow from the HMEM forum have been doing a lot of research to find a small I.C. carb with a throttle on it. Hit and miss engines do not have a throttle on them, as the speed is controlled by the governor system. There is no throttle in those carbs--they run "wide open" all the time, as anyone can attest to who has ever had the misfortune to have the governors stick on their hit and miss engines. However, not all of the model engines we build are hit and miss style. Now don't get me wrong.--There are lots of carburetor designs out there for I.C. engines, but they are HUGE compared to the size of the engine itself. We have been searching for carbs that won't look out of place on engines from 1/2" to 1" bore with a single cylinder. After looking at many different plans, we have decided that the best design out there is a design originally put together by George Britnell for use on his 4 cylinder engine. The only fly in the ointment is that George's carburetor has a relatively large bore in it at 0.260" diameter. On the small I.C. engines that I build, they depend on air flow thru the carb venturi to create enough vacuum to pull fuel up to the carb from the fuel tank. If the bore of the carb is too large, the resulting vacuum created by too large a carb bore compared to the engine displacement won't create enough venturi vacuum to suck up the fuel to the needle valve spray "nose". I have taken Georges carburetor and redesigned it to accommodate a bore of 0.156" in the venturi area. This has allowed me to make the overall carb body smaller and to make the attendant air bleed screw and idle speed screw smaller as well. George knows I am doing this, and has even offered up some helpful advise. I am going to build this carb, and use the Webster engine I built a few years ago as my "test bed". Stick with me, and I will post detail drawings as I progress. It will probably be safer to wait until the end of the build to actually download any drawings, because they may change as this thing develops.----Brian


http://s307.photobucket.com/user/Br...w013/FULLASSY-RUPNOWCARB_zpsacdb7dd1.jpg.html


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## Brian Rupnow (Mar 11, 2014)

I'm not certain if this is the most difficult part or not. It certainly seems to have the most machining operations on it, so we will start with the outer body.


http://s307.photobucket.com/user/Br...Rupnow013/RUPNOWCARBBODY_zps26a5be00.jpg.html


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## Brian Rupnow (Mar 11, 2014)

Well, I think that covers everything I am going to do in the lathe. Started out with 3/4" square brass, mounted it in the 4 jaw, and turned the big end spigot to finished diameter but about 1/2" long. Put 3 jaw on chuck, reversed the part and held it by the round spigot to turn the other end to 5/16" and left it about 1/2" long. Threaded o.d. of small spigot and drilled 3/16" dia. hole thru all, then reversed part and put threaded spigot in 3 jaw. Drilled 0.260 dia half way thru from big end, then faced big end off to correct length and bored taper as called for on drawing. Removed part from lathe, ran a 0.2" thick nut all the way down the threaded end and ground off anything sticking out past the nut away on my stationary belt sander.


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## Brian Rupnow (Mar 11, 2014)

Well There!!! That wasn't so bad. The biggest worstest part is finished. Its too bad that my camera picks up all the filings as well as the part I am trying to show. It certainly is a tiny little rascal.


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## Brian Rupnow (Mar 12, 2014)

And the next part of our saga looks like this. I decided to go with steel for this part, because I was afraid that aluminum or brass wold "gall" and seize in the brass outer body. (It has to turn freely). When I put the 0.375" hole in the brass body I used a 3/8" endmill, so my hole is a couple of thousandths oversize. I will turn this part from 1/2" dia. cold rolled steel, because round cold rolled in its stock form is about .0005" undersize and I don't want a sloppy fit. I know the drawing calls for 0.375" diameter but I will probably end up at .376 when I make the part.


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## kuhncw (Mar 12, 2014)

Brian, thanks for posting your work on this downsized carb.  I've been wondering about a good combination of materials for the barrel and body.  Steel is worth a try.

Regards,

Chuck


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## Brian Rupnow (Mar 12, 2014)

So---Now we have two parts. You may notice that the top of the barrel doesn't quite agree with the drawing. As I was about to cut it to length I had a revelation---The single bolt which holds the throttle handle on is right on the center, and the handle is sure to spin on that bolt instead of turning the barrel. What better time to make the barrel a shade longer and put a 1/4" slot on the top. Align the slot with the hole thru the barrel and I have a quick visual reference of whether the throttle is open or closed. The handle will fit down into the slot.  I am going to post an updated barrel drawing as soon as I finish this post. That Canadian quarter in the picture is the same size as an American quarter.--A really nice feature of this carb (Thank you Geo Britnell) is that the #2-56 bolt that interacts with the slot in the barrel has two functions. It acts as a throttle stop, to adjust how far the throttle can close before being stopped by the end of the bolt, and the side of the same bolt holds the barrel in place so it can rotate but not fall out.---Very nifty "dual purpose" design. The hole thru the barrel is 0.156" diameter. As for the end view---If that hole in the barrel doesn't appear to be perfectly concentric with the hole in the carb body, well----That is a testament to the fact that I am much further advanced in my design capabilities than I am in my machining abilities. A sharp eyed individual will also note that my taper in the intake side of the body doesn't nicely taper to a matching diameter with the hole thru the barrel. That is because of constraints imposed by the .040 diameter hole thru the side of the carb body which is part of the idle circuit. The law that governs how a venturi in a carburetor creates a low pressure area where there is a restriction in the airflow, (to suck fuel up hill from the gas tank) doesn't explicitly state that it must be a smooth and seamless transition from the large diameter passage to the small diameter passage.--Also, the nose of the needle valve outer shell is going to stick up in the center of that 0.156" diameter, causing a further restriction in the air passage. If there is a problem, I will machine a nicely tapered insert and put it in the intake side of the carb body.


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## Paulsv (Mar 13, 2014)

Great drawings, and great work, Brian.  Thanks for doing this, and I'll be following all the way!


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## Brian Rupnow (Mar 13, 2014)

We have to get some fuel into this thing, and the fuel jet is made up from two parts soldered together. No real rocket science here, but I did have to call and order $100 worth of 10-40 taps and a 10-40 die.











http://s307.photobucket.com/user/Br...2/SUBASSYRUPNOWFUELINLET_zpsa140dde8.jpg.html


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## Brian Rupnow (Mar 13, 2014)

The needle valve itself is of two piece construction. There are no threads on the fabricated "needle"--The threads are on the inside of the cap.


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## Brian Rupnow (Mar 13, 2014)

I had asked George Britnell a question about the location of the air bleed hole location and throttle opening here, but now realize that George had answered me in a previous email. It just took me until this afternoon to get my head around what he was saying.


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## RonGinger (Mar 13, 2014)

Why do you feel its worth the expense to get 10-40 threads? Wouldn't 10-32 be fine enough? Thats a standard that any hardware store has.


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## Brian Rupnow (Mar 13, 2014)

-Every set of carburetor plans I have ever seen ask for 40 tpi pitch on the threads that control the needle valve. I have always used  taps with 32 tpi. I can only assume that the added control of fuel flow that you get from those extra 8 threads per inch must be very important, as all the carburetor guys use them in their plans. I may also be a gullible fool.-I have decided that anything I can do to make my carbs work better is probably worth spending the money on. Only time and experimentation will tell.


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## Brian Rupnow (Mar 13, 2014)

I guess if you are going to have a throttle, you will want a handle to turn it with.


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## Brian Rupnow (Mar 13, 2014)

I know, I know---I said to make the handle out of steel. But when I got to my "cupboard of all bits and pieces" the only 1/16" stock I had was brass. I don't think you can bend brass too readily without it breaking, so I heated it up with my oxy acetylene torch till it was dull red and then bent it in my vice. I'm somewhat amazed at how few pieces there are to this carb. All that's left to make is the fuel inlet and the needle and cap. I might solder up the fuel inlet tomorrow and get it ready for when my taps and dies arrive.


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## Brian Rupnow (Mar 13, 2014)

Okay---At this stage of the game, I am beginning to realize that ---I muffed it!!! On the carburetor main body, the "thru-hole" that the air flows thru has to be the same diameter both entering and leaving the round central chamber where the throttle barrel is.--Otherwise, as I have just figured out (much to my dismay) the amount of opening or closing on one side of the barrel (for instance the inlet side) is totally different than the amount of opening or closing on the opposite side. I am going to have to reconfigure the main carburetor body--as in redesign and remake it. That's okay. I'm learning as I go along here. I think I can save the barrel and the handle.


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## Brian Rupnow (Mar 13, 2014)

There wasn't a lot of redesign involved. I made the hole thru the air intake side the same diameter as the hole in the air outlet side at the point where it breaks into the 3/8" hole that the throttle barrel fits into, and moved the air bleed hole and screw around a bit to optimize their location according to advise from Geo. Britnell.---George had sent me this information earlier, but I couldn't really get my head around what he was saying until I read thru it again today while I was puzzling things out. I can re-use the barrel and handle.--Interestingly enough, this throttle goes from closed to open in 45 degrees, unlike the butterfly valve in an automobile carb that must rotate thru 90 degrees.


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## Brian Rupnow (Mar 14, 2014)

This morning, before diving headlong into another piece of brass, I am doing a bit of taper experimenting. I wanted to know if I can drill a 20 degree included angle taper thru material 0.361" thick and go from a diameter of 0.315 to a diameter of 0.187". My selection of small boring bars is, well, minimal would be a good word. I didn't want to have to make up a special fluted taper cutter. I reproduced the critical geometry in aluminum--a 3/16 centered hole going .361" before breaking thru to a 3/8" diameter cross hole. I found that by drilling a couple of stepped holes first to remove as much meat as I could from the outside end of the aluminum piece without cutting into what would become the tapered hole, I was able to get my much abused boring tool thru and give me the correct taper and correct diameters. That's a big plus!!! Now I am off to my supplier to buy some more 3/4" square brass. The outer body could be made from aluminum, but I like the look of brass better.


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## Brian Rupnow (Mar 14, 2014)

It's been a looooonnnng day!!!  I have remade the brass outer body to the new design, and it looks good.---But---I just found out I can not re-use the barrel. This was a case of two wrongs not making a right. When I made the first brass body, I got something off center. Scratched my head for a minute and thought--"Oh well, I'll just drill the hole thru the barrel off center a bit so it will match up".---BAD MOVE!!! Now the barrel with the off center hole won't work with the new brass body. OH POOP!!! I'm going upstairs and read for the rest of the day. Fresh start tomorrow!!!


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## Brian Rupnow (Mar 15, 2014)

Nothing picture worthy accomplished today. I spent the whole day messing with sump pump stuff. I wasn't totally impressed with the 20 degree included angle tapered hole that I had machined on the inlet side of the new brass body. So--I turned a 20 degree included angle taper on a piece of 3/8" cold rolled and used some really coarse, nasty automobile valve grinding compound on it to hand finish (can't honestly say "lap" ) the inside of the taper. It turned out quite nice actually. I don't really have any boring tools small enough to bring this off properly.


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## Brian Rupnow (Mar 16, 2014)

Progress has come to a stop. The new brass outer body is finished. In the process of making the new barrel, I convinced myself the safest way to ensure concentricity was to insert it firmly into the brass outer body and drill it in place. This worked fine for the 0.156" diameter thru hole. I then rejigged the part and went to spot thru the #2-56 threaded hole for the idle stop screw so I would be able to get the slot in the correct place. I opened the drawer with my 1/16" end mill in it, and guess what!!! The Shop Pixies have taken it away. I don't want to tear down my set up, so everything will set until tomorrow when I can get over to the tooling shop and buy a couple of new end mills. I can't spot thru with a drill, because it is not on the center of the 3/8" diameter steel, so a drill would slide sideways and tear up the threads in the brass part.


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## necchiom (Mar 16, 2014)

Great job Brian! With compliment and thanks for your contributions.


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## Brian Rupnow (Mar 16, 2014)

Thanks Necchiom---I find this very fascinating, even when I screw up and have to make everything a second time. Thankfully, that doesn't usually happen.----Brian


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## gus (Mar 17, 2014)

Plan to make this Carb for my Webster IC Engine. 

I have exhaust valve collision with piston when H&M cut-in. Just figured out a fix.


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## Brian Rupnow (Mar 17, 2014)

Today we are back to the point we were at when I figured out that my first design had a fatal flaw. The carb outer body is remade, and the steel center barrel is remade as well. What a pin in the butt!!! One pic shows the assembled carb setting up posing for an "overall" shot, one shows it with the throttle closed. (The closed position is adjustable) The throttle never closes completely, otherwise the engine would stop. It is only when the carb is closed to the position showed that the air bleed screw comes into play.--And remember, since the openings in both sides of the brass barrel are the same size on each side of the throttle barrel, the amount of opening on the side closest to the cylinder head will be open exactly the same amount as the opening on the air intake side. If the engine falters when the throttle is in this closed (idle) position shown,  then the air bleed screw can be adjusted to smooth out the idle. Once the throttle is swung into a more open position, the idle circuit is blocked by the side of the barrel and has no effect on the carburetor.


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## Brian Rupnow (Mar 17, 2014)

This is the best shot I could get of the carb with fully open throttle. What appears to be a bit of dirt right at the top of the barrel bore is actually a bit of metal displaced by the drill when I tapped the hole for the bolt holding the handle on. I will clean that up and plug it with a bit of thread sealant on the handle bolt.


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## Brian Rupnow (Mar 17, 2014)

What you see here is a temporary disposable jig, made from aluminum. There is simply no other good way that I know of to position the two brass pieces correctly in relationship to each other, and then not have them move while silver soldering them. The jig is "temporary and disposable" because it has to be cut apart to get the soldered assembly out of the jig. The short piece of brass is for the fuel line to attach to, and has been turned to its finished size, and the hole drilled part way thru the center of it. The longer brass part is 1/4" diameter and will be turned down to finished size on each side of the fuel inlet before being threaded #10-40 on both ends. At some point while it is in the lathe chuck, the tapered end will be machined and the thru hole will be drilled. After that, the hole part way thru the short piece will be extended until it intersects with the central fuel passage.


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## Brian Rupnow (Mar 17, 2014)

This is the result, immediately after silver soldering the joint. I hold the aluminum in an old drill press vice so I can position it in whatever way I have to so I can reach all areas of the soldered joint.


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## Brian Rupnow (Mar 17, 2014)

And here we have the soldered part, freed from the jig and cleaned up a bit. Now I get to do some more lathe work on the main 1/4" diameter part.


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## Brian Rupnow (Mar 17, 2014)

I've been holding my breath for the last two hours machining this fuel inlet, but I think I've got it nailed. I don't know why I hold my breath while I'm doing these really finicky things, but I bet you do too, don't you!!!


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## Brian Rupnow (Mar 18, 2014)

WOW!!! I didn't think I was going to be able to do this. That small diameter is 0.050". The smallest thing I had ever machined before was 0.093" dia. valve stems. I used a 3/16" cold rolled rod, and put the taper on the end while the rod was still at full 3/16" diameter. Then I started turning at 1000 rpm for 1/8" at a time right down to 0.050, then move ahead and machine the next 1/8" down to 0.050. I followed the same procedure to turn the 0.100" diameter. I tried to keep all machining as close to the chuck jaws as possible. I'm not even going to try to part it off. I will use the hacksaw while it is still locked in the chuck to cut it off, then use my belt sander to cut it to length.--To keep from feeding the swarf bunnies, I will wrap a length of masking tape around the part before I cut it off.


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## Philjoe5 (Mar 18, 2014)

Nice job on a tricky part.  



> That small diameter is 0.050"





Cheers,
Phil


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## Brian Rupnow (Mar 18, 2014)

There we go---All finished. I cheated on the locknut---took a #10 std. pitch nut and rethreaded it. The soldering of the needle valve went very well, with no miss-haps. I have much better luck with silver solder than I do with plumbers soft solder. I coated the knurling on the needle valve with white out to keep any overflows of silver solder from sticking in the knurling. It works a treat, but I had a devil of a time getting the white-out off after the fact. I tried hot paint thinners, varsol, dishsoap, Goo-Gone, and toothpaste and hot water, all scrubbed with an old tooth brush, and I think there is still some on there. The drawings all have to be updated and changed to reflect what actually got built, but I will do that later today. Damn, it's a lot of work when you have to make the two main parts twice!!!--The $64,000 question will be now---Does it work!! As soon as the weather gets nice enough that I can work out in my main garage without freezing to death, I will be testing this carb and Chuck Fellows' carb on my old tried and true Webster, and will try and give an unbiased report and a video of the results.---Brian


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## Brian Rupnow (Mar 18, 2014)

As of this evening, 18-March-2014, I have replaced all of the drawings in this build thread with the most current drawings. They are all up to date now.---Brian


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## Brian Rupnow (Mar 18, 2014)

This is a picture showing the size difference between the carb I just built based on a modified George Britnell design, on the left, and one I built a few weeks ago to Chuck Fellows design on the right.


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## gbritnell (Mar 18, 2014)

Hi Brian,
Congrats on the carb build. It looks great! I see no reason why it shouldn't work. Start with the air bleed open half way. With the needle barely open start turning the engine over. Choke once with finger, if it doesn't pop open the needle a touch more and repeat.
gbritnell


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## Brian Rupnow (Mar 18, 2014)

Thank you very much George. I take that as a great compliment. All I need now is some warm weather to get out in the main garage and test it.--Brian


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## gus (Mar 18, 2014)

Brian Rupnow said:


> Thank you very much George. I take that as a great compliment. All I need now is some warm weather to get out in the main garage and test it.--Brian




Hi Brian,

When winter comes,spring is not faraway.( Weekend just three days away to hook up some slimy big fish.)
Making some progress on the H&M device.Just take my sweet time putt-putting your engine. Could spent the whole morning enjoying the putt-putts.

Take care.


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## Brian Rupnow (Mar 19, 2014)

This is the Webster engine I will be trying the two different carburetors on. As you can see, it already has an adapter on it to hold the Traxxas Pro 15 model airplane carb on it. (the Traxxas has a 10mm plain outer barrel diameter.)



 This is the adapter I will make today so the two different carbs can be tested. The small diameter of the adapter must be Loctited into the Webster intake manifold.


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## Brian Rupnow (Mar 19, 2014)

A very minor change in direction this morning- I realized that with the air bleed adjustment screw on the bottom of the carb as originally planned, it would be difficult to get at and adjust while the engine was running. So--I drilled and tapped the #2-56 hole completely through to the top of the carb body and plugged the bottom of it with a short #2-56 shcs. Then I made a very small notch for clearance in the handle, machined up a knurled adjustment knob, and threaded it, then Loctited it to a #2-56 screw x 3/4" long. Now I can adjust it from the top side where it is easy to reach. Also in these pics you can see the adjuster which will let me attach this carb or Chuck Fellows' carb to my Webster engine.


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## Brian Rupnow (Mar 19, 2014)

Before I try and run the Webster with my carburetor or Chucks carburetor on it, I thought it would be a good idea to start it up and make sure it ran okay with the Traxxas Pro-15 model airplane carb on it. Its a good thing I tried, because the ignition points on it had gotten damaged somehow. I replaced the points (You can see the box setting in the background beside my Power Box.) The movie kind of says it all------Brian.


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## Philjoe5 (Mar 19, 2014)

Brian,
Good luck with the new carbs.  Fingers crossed

Cheers,
Phil


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## Brian Rupnow (Mar 20, 2014)

So--Here we go with the first run of what I'm going to call the Rupnow/Britnell carb. The engine runs. The throttle works. The needle valve is VERY touchy, and I'm going to have to put a spring around the outer body to bear against the needle adjusting screw because it rapidly screws itself in or out depending on the engine vibration. Right now I can't see that the air bleed screw makes much difference, but I haven't ran it long enough to make that judgement quite yet. I was afraid this morning when I assembled things for the final time that it might leak fuel at the bottom of the carb, because the of the needle valve going in from the bottom, but that doesn't seem to be the case. You will notice in the video that I have a small piece of cardboard setting directly below the carb to show up any fuel drips.--However, it doesn't appear to be dripping any. I have a few more things to do today, and then later today or possibly tomorrow I will put Chuck Fellows carb on and test it.---Brian


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## Brian Rupnow (Mar 20, 2014)

As promised, a video of the Chuck Fellows carburetor running on the Webster, and a bit of a surprise.


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## Brian Rupnow (Mar 20, 2014)

If I have done this correctly, this link should get you all of the drawings as .pdf files.---Brian
http://www.mediafire.com/download/cjgmg9kj2vcgy70/RUPNOW-BRITNELL_CARB.zip


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## Philjoe5 (Mar 20, 2014)

Brian,
Fantastic work and great videos.  Thanks to you and Chuck.  I agree, who could want better speed control than what you get from either yours or Chucks designs.

Are you using gasoline or Coleman fuel?

Cheers,
Phil


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## Brian Rupnow (Mar 20, 2014)

I use Coleman fuel. Gasoline stinks too much for me. By running about a 30:1 mix with 2-cycle engine oil you don't need a cylinder oiler either.


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## cfellows (Mar 20, 2014)

Great test, Brian!  Needless to say, I'm pretty tickled with the results.  While watching your progress on the Britnell carb, I've also been doing further testing with my original design, although I haven't posted anything.
 I learned a couple of things.  

 First, the rotational position of the needle valve assembly is very touchy.  The fuel aperture into the air passage should point directly at or slightly in front of (away from the engine) the throttle screw.  This makes a considerable difference in the way the carb performs. 

 Second, it turns out the problem I was having wasn't the fault of the carburetor, it was the ignition I was using.  As you recall, I was testing the carburetor on my vertical single and I hadn't really shaken all the bugs out of the engine itself.  So, with a proper working ignition installed, the results I'm getting are much better.  I built another carburetor for the vertical single, sticking to my original design, but I made the venturi 5/32" instead of the original 1/8".  I also used a 10-32 throttle screw instead of the 8-32.  This carburetor works quite well and I'm happy with it.

 I'm still using my first carburetor with the 1/8" venturi on my plumbing parts engine and it runs flawlessly.  It will idle down to a very slow speed and never miss a lick.

 So, in summary, I was a bit quick on the trigger to abandon my original design and your test seems to confirm my later results.  I think both your carb and mine will satisfy most needs for slow running model engines.
 Thanks for all the work you put in on this, particularly in documenting your build and in running the comparative tests!

 Chuck


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## Brian Rupnow (Mar 22, 2014)

Okay--Small things amuse small minds. (Nah, that isn't really true). We've got 6" of new snow here and its still snowing, so I decided to play indoors today. I wanted a throttle handle on Chucks carburetor, so today I built one. Now if you immediately ask "Why not just drill a cross hole through the head of the #8 shcs that acts as a throttle?", there is method in my madness. We don't really know what rotational position the head of the screw will be in at the desired low rpm. I wanted to put a stop pin in for the throttle lever to stop against, so I had to be able to adjust the position of the lever in relationship to the position of the #8 capscrew.


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## Brian Rupnow (Mar 22, 2014)

If you look close, you can see that I just had room to drill one corner of the carb body for a 1/16" diameter "stop pin" that I Loctited into place.


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## Brian Rupnow (Mar 22, 2014)

And here are the sordid details of how I made it----


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## Brian Rupnow (Mar 22, 2014)

This is a view of the carb from the far side, which shows the "stop pin" for the throttle arm. I just had enough room to mount it in the corner of the carbs rectangular body. The throttle arm is in the "fully open" position in this picture, but when it swings up against the pin and stops, that is the idle position.


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## gus (Mar 23, 2014)

Hi Chuck and Brian,

OS Carbs cost an arm and a leg. Been looking for simple and effective DIY Carbs. Will build both carbs and put to into my inventory. 
Both carbs were very effeccive and responsive.

Hi Brian, Trust the snow melt is not a problem yet.Meanwhile I am enjoying putt-putting your engine. No worry.Gus will fix the exhaust valve/piston collision when switching on H&M. I goofed on some TDC.


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