Sizing carburetor for model engine

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awake

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I'm building a Webster - .875" bore, 1.25" stroke. I was thinking I would make a carburetor along the lines of the one designed by Chuck Fellows, but I'm seeing a couple of variations, one with a 1/8" through hole and one slightly larger at 5/32".

I have no idea what size of carburetor is appropriate for an engine of this size. Any helpful pointers will be most welcome!!
 

Smokey Joe

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I too am interested in hearing what other people use as a rule of thumb for sizing carb to engine. One of the guys I talk to about it said that the venturi should be 10% of the size of the cylinder diameter for these small engines. So that would mean if you have a 1 inch bore your venturi should be .100 inches.
 

Longboy

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I use model airplane engine carbs. Traxxas & O.S. for .10-.15 engines. Work well on the 11cc singles to my 65cc four cylinders, fast idle speeds. The Webster ran nice on a vapor carb with a 3/16 in. air feed hole.
 

Brian Rupnow

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Traxxas Pro 15 carburetor works well on Webster.
 

awake

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Thanks, all. Brian and Longboy, I'm hoping to succeed with a home-built carb. I realize this is adding another variable to the uncertainties of getting my first IC engine running, so I have the Traxxas in mind as a back-up plan.

Brian, I seem to recall seeing a post where you tried Chuck's carb on your Webster, with success. Do you recall which version (which throat diameter) you used?

XD351, very interesting formula. (And an app - I never would have thought to look for such!) I tried out the formula, and realized that RPM is a dominating variable with an engine this small.

.875" bore x 1.25" stroke = 12.32 cc, rounded to 2 decimal places, if I have done the math right.

Here is the chart, showing the lower and upper ranges (K = .65, K = .9) at assumed RPMs of 1000, 2000, 3000; results are in inches of diameter:

Screenshot from 2020-02-23 09-56-34.png


In the two variations that I've seen for Chuck's carb, the throat is either 1/8 = .125" or 5/32 = .156". Looks like .125 would be more appropriate if the RPM tops out at 1,000; .156" might be ideal for a top RPM of 2,000 to as much as 3,000.

So ... for an engine like the Webster, what is an appropriate upper limit for the RPM? For that matter, what is a typical RPM at idle?
 

Brian Rupnow

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My i.c. engines typically idle at around 900 to 1000 rpm. Power band is at 1600 to 2500 rpm. They will rev higher than that, but I don't let them because they are a lot of work to build and I don't want them to fly apart.
 

awake

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Thanks. I'm glad I was in the right ball park! With that in mind, I'm thinking about splitting the difference and drilling the throat at .140".
 
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