# Model Carburetor



## cfellows (Mar 31, 2010)

Never happy to leave well enough alone, I decided to build a small model carburetor for my Plumbing Parts engine. This design is similar to a larger version I built several years ago although I never tried that one out so I'm hoping this one will work. This is a pretty straight forward design, similar to some model airplane carbs.

Started out with a lump of brass, 7/16 x 7/16 x 1 1/8".







Here it's chucked in a 5C square collet and I'm turning down the outboard end of the main body.






And here's the outboard end turned to the finished OD.






I turned the body around and here is the inboard end turned to the finished OD of 1/4"






Here I'm center drilling the 1/8" hole that will be drilled completely through the carburetor body.






Here I'm fixin' to put a 1/4" x 40 model pipe taper thread on the inboard end. This end will thread into the intake manifold.






Here you see the threaded end. I've also cross drilled the 1/4" thru hole that will house the throttle body.






Speaking of the throttle body... here it is. Started out with 7/16" brass rod and turned it down to 1/4"






Here you see the carburetor body slipped over the turned end of the throttle body. This is mark the position of a groove which will hold a quad o-ring which will hold the throttle body in place.






Next I drilled a #51 hole axially through the center of the throttle body. It will be threaded 2-56 on top to receive the needle valve which will be made from (you guessed it) a 2-56 socket head cap screw.






Here I've got the throttle body inserted into the carburetor body and I'm getting ready to cross drill a 1/8" hole radially through the throttle body, using the existing 1/8" hole in the body as a guide.






Probably obvious to most folks, but looking through the carburetor body, you can see the hole in the throttle body when it is in the "wide open" position.






And here you see it with the throttle body partly turned which partially closes the opening through the carburetor body.






We're in the home stretch now. Here is a picture of the finished carburetor body, the throttle body, the quad o-ring that will hold it together, and the needle valve.






Needle valve threaded into the throttle body.






And throttle body with needle valve inserted into the carburetor body.






The throttle body also has a 1/8" hole coming up from the bottom. This will have piece of 1/8" rod with a very small axial hole, probably around .040", soldered in. The piece will extend part way into the radial center hole of the throttle body and will be the seat for the adjustable needle valve. The other end of the 1/8" rod will extend maybe 3/8" out the bottom of the throttle body where the fuel line will slip over it. More pictures of that part when I get it finished.

Chuck


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## Speedy (Mar 31, 2010)

!!! 
That is looking like a very nice carburetor! 
thank you for posting this, I always wondered what is involved in such a build.
cant wait to see you testing it out on an engine ;D


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## gbritnell (Mar 31, 2010)

Hi Chuck, You went with a rotatable barrel on this carburetor. Are you planning on changing the engine speed with the carb? If not, you could have simplified the carb by just machining the venturi into the body. If so, then you will need some type of air bleed on the carb to lean out the mixture at low rpm's. When the needle is adjusted for clean running at higher rpm's and the throttle barrel is closed the vacuum signal gets very strong and draws too much fuel at an idle hence the need for the air bleed. 
 I apologize if you already know this but with all the carb work I have done in the last year or so I have learned quite a bit about them and I would hate to see you getting frustrated trying to tune one in. 
George


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## radfordc (Apr 1, 2010)

Where does the fuel enter the carb? Normally, as the throttle body rotates and meters air, it must also meter fuel to keep the mixture constant.

Charlie


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## cfellows (Apr 1, 2010)

radfordc  said:
			
		

> Where does the fuel enter the carb? Normally, as the throttle body rotates and meters air, it must also meter fuel to keep the mixture constant.
> 
> Charlie



Charlie, the fuel will enter the carb through a 1/8" OD tube that will be soldered into the bottom of the throttle body. I will adjust the fuel mixture with the needle valve on top at different speeds.



			
				gbritnell  said:
			
		

> Hi Chuck, You went with a rotatable barrel on this carburetor. Are you planning on changing the engine speed with the carb? If not, you could have simplified the carb by just machining the venturi into the body. If so, then you will need some type of air bleed on the carb to lean out the mixture at low rpm's. When the needle is adjusted for clean running at higher rpm's and the throttle barrel is closed the vacuum signal gets very strong and draws too much fuel at an idle hence the need for the air bleed.
> I apologize if you already know this but with all the carb work I have done in the last year or so I have learned quite a bit about them and I would hate to see you getting frustrated trying to tune one in.
> George



Thanks George. I did know that there is more to carburetion than my humble beginning features. However, this is mostly experimental and I don't plan to vary the speed of the engine much. I just wanted a way to try different speeds and to adjust the fuel mixture as needed at different speeds to make it run smoothly.

Chuck


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## putputman (Apr 1, 2010)

Chuck, I don't mean to hijack your post, but I am wondering if George has throttle controlled carburetor design (and would be willing to share it with us)that would work on most of these small engines. I have a couple Atkinsons, a Webster, & some other engines that have simple fuel mixers on and would like to improve with a decent carburetor. 
Perhaps he could start another post if he has one.


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## capjak (Apr 1, 2010)

Thanks for another great tutorial Chuck. I learn so much from every one of your posts.

Jack


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## cfellows (Apr 1, 2010)

putputman  said:
			
		

> Chuck, I don't mean to hijack your post, but I am wondering if George has throttle controlled carburetor design (and would be willing to share it with us)that would work on most of these small engines. I have a couple Atkinsons, a Webster, & some other engines that have simple fuel mixers on and would like to improve with a decent carburetor.
> Perhaps he could start another post if he has one.



No problem, Arv. I welcome any and all information relating to carburetors.



			
				capjak  said:
			
		

> Thanks for another great tutorial Chuck. I learn so much from every one of your posts.
> 
> Jack



Thx Jack. However, probably need to see if this thing works before anybody gets too excited!  

Chuck


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## cfellows (Apr 1, 2010)

Here's a photo of the finished (for now) carburetor. As you can see, I have soldered a 1/8" piece of rod into the bottom of the thottle body. This 1/8" piece of rodis 5/8" long and has a #51 hole drilled from the bottom up to about 1/8" from the top. The remaining 1/8" is center drilled to about .030". The fuel line is attached to the bottom of this rod.






Here is a view into the venturi end of the carburetor. You can see the tip of the 1/8" rod extending a little less than half way into the passage. You can also see the needle valve coming down from the top. The fuel mixture is adjusted by screwing the needle valve toward or away from the hole in the tip of the jet coming up from the bottom.






The engine speed is determined by turning the throttle body. Unfortunately, that is also a problem with this design. Since the fuel line attaches to the bottom of the throttle body, turning it also turns the fuel line, and, since it is made out of a rubber like material, resists turning. In fact, it acts like a spring and tends to hold the thottle body in one position. So, this carburetor, as designed, is not going to work!

Chuck


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## stevehuckss396 (Apr 1, 2010)

You could keep the throttle still and twist the engine!! Haahahha!!


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## Deanofid (Apr 1, 2010)

It sounds to me like you have a built in return spring, Chuck. Seriously.
I've made a couple of this type, but, not knowing what I was doing, had a hard time getting them to run consistent. 
Yours looks very nice, and I hope it works out for you.
Could you cut a small groove in the 'handle' part where it meets the body, for a thin o-ring that would prevent the fuel tube from twisting the throttle body?

Dean


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## doc1955 (Apr 1, 2010)

stevehuckss396  said:
			
		

> You could keep the throttle still and twist the engine!! Haahahha!!



 :big: :big: :big: :big: :big:
Seriously I just received plans for a carb and here is one in work.
And I must say looking pretty nice.


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## cfellows (Apr 5, 2010)

For those interested, here are drawings for the carburetor I made for my plumbing parts engine. I'm pretty happy with it's performance, but it does require some adjusting. Although it will run your engine at various speeds, it's mostly designed for engines to run at one speed most of the time. It will let you adjust your engine to run at idle speeds. Let me know if you have questions or the drawing is missing anything. I will upload a PDF file in a few days after I correct any missing or wrong info.

For some photos of the carburetor, visit this link:

http://www.homemodelenginemachinist.com/index.php?topic=6352.0

Chuck 

View attachment Visio-CarburetorDrawings.pdf


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## lathe nut (Apr 6, 2010)

Chuck, you sure do good work, been getting material to build an IC engine but a few things have held me back one was the Carburetor I do understand how they work but building one was a different story, you make it seem so simple, but getting it done is another story, thanks for some of you fellow that share with us that do not know, Lathe Nut


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## kustomkb (Apr 7, 2010)

Nice job Chuck!

Thanks for sharing the drawing.


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## maintman (Jan 9, 2014)

where can i get a copy of the plans for the brass carburator


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## kadora (Jan 9, 2014)

I can not see pictures in this thread WHY ???


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## Gordon (Jan 9, 2014)

Chuck: How do you hold the spool into the body? I follow the drawings but it looks like the spool is not retained>


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## cfellows (Jan 9, 2014)

Gordon, the throttle barrel just sits in body.  Not a great design, but I never had a problem with it trying to come out.

 Kadora, not sure why you can't see the pictures.  I can see them OK...

 MaintMan, look further down, I think reply @13 and there is a PDF file.

 I have come up with a new design that's simpler to build and works better.  See my thread on my vertical single engine where I'll be posting pictures and plans for my new carb.

 Chuck


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## Gordon (Jan 10, 2014)

Chuck. That is interesting because I seem to have trouble with all of my carburetors being too sensitive. 1/16 turn between running and not running. With the spool floating the adjustment would be very variable.


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## cfellows (Jan 10, 2014)

Gordon said:


> Chuck. That is interesting because I seem to have trouble with all of my carburetors being too sensitive. 1/16 turn between running and not running. With the spool floating the adjustment would be very variable.


 
 My design is sensitive as well.  But my throttle barrel fit pretty snug, so it wouldn't move around from vibration.

 In retrospect, I really don't like the barrel throttle design.  While simple in design, it's kind of big and clunky.  My newest design uses an external  throttle sleeve and it's easier to see it's position.  I do think I will try some refinements so the throttle isn't so sensitive.  Here's a link to my latest thread which has some pictures and a drawing.

http://www.homemodelenginemachinist.com/f31/vertical-single-4-stroke-ic-engine-21383/

 A problem with small engines is that they really don't require a very big venturi and/or throttle opening.  My newest design has a venturi less than 1/8" and could very well be smaller in my opinion.

Chuck


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## gbritnell (Jan 10, 2014)

As Chuck said, smaller is better. The only thing that will happen if the venturi is too small is the engine will run out of air so therefore limiting the rpm. It's easier once you make the components to enlarge them as needed. There's a fine line between the engine's operating rpm range and the size of the venturi. Usually if the engine is slow running like a hit and miss you can make the venturi smaller to get a greater pressure drop at the needle. The nesxt thing is to put the finest thread you can on the needle valve. This will help reduce the sensitivity. Most of my engines that have a variable rpm range use rotating throttle barrels. I did build one of the Jerry Howell designed carbs and it works but is a little more complicated to build.
gbritnell


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## AussieJimG (Jan 19, 2014)

Your thread(s) have me thinking about carby design and I have built my latest one using your offset spraybar principle. 

After breaking several tiny drills (0.5mm) while trying to drill the fuel jet, I am wondering if the small fuel jet is needed at all. My thinking is: as the needle goes past the venturi hole, it partially blocks it thus doing the same job as when it blocks the fuel jet.

I can't test the theory yet as my current "engine" is still in doorstop mode even with a vapour carby on it. 

Am I totally round the bend?

Jim


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## cfellows (Jan 19, 2014)

AussieJimG said:


> Your thread(s) have me thinking about carby design and I have built my latest one using your offset spraybar principle.
> 
> After breaking several tiny drills (0.5mm) while trying to drill the fuel jet, I am wondering if the small fuel jet is needed at all. My thinking is: as the needle goes past the venturi hole, it partially blocks it thus doing the same job as when it blocks the fuel jet.
> 
> ...


 
 Jim, on my last carburetor, I avoided the really small hole by using 1/16" brass tubing, which has a .030" ID.  I drilled a #55 hole all the way through, then came up from the bottom with a 1/16" drill.  Then I soldered a short piece of the 1/16" tubing in place to serve as the needle seat.

 Chuck


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## AussieJimG (Jan 20, 2014)

Good thinking Chuck, much better to buy a hole than to make one. But I am still wondering about the need for it in a spraybar carby. 

And still trying to coax life from the doorstop to check.

Jim


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## cfellows (Jan 20, 2014)

AussieJimG said:


> Good thinking Chuck, much better to buy a hole than to make one. But I am still wondering about the need for it in a spraybar carby.
> 
> And still trying to coax life from the doorstop to check.
> 
> Jim



 Can't really say whether just the needle closing off the aperture would work or not.  I do know that with the seat in place, the adjustment is really sensitive with less than 1/8" turn making the difference.  C'mon, Jim, get off the couch and go give it try! 

 Chuck


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