Carburetor question

[Gordon]

I remember seeing a rule of thumb regarding the ratio of carburetor bore diameter to cylinder bore but I do not remember what it was. I am currently working on an engine for the 1/8 scale Oil Pull Rumely I am working on. I have a 1 1/4 bore and I put on a carburetor which I had made some time back. It had a 1/8 bore and I could get the engine to sputter but not actually run. I enlarged the bore to 5/32 and I can get the engine to run for about 30 seconds with the throttle at minimum opening and then quit. It also quits if I open the throttle. Brian Rupnow just completed a 1" bore engine and he used a commercial carburetor with 6 MM bore.

Before I resize my present carburetor or make a new one does anyone have any thoughts on correct sizing? Should I be looking at orifice size instead of bore?

Gordon

 

[Vietti]

Some folks put a strip of tape over the carb mouth and then make larger and larger holes in it till they find a size the engine likes.

 

[coulsea]

What design is the carburetor?.
5/32 or 4mm should work on an engine that size but there are many factors that can affect it.
enlarging the bore will reduce the effectiveness of the venturi by slowing the speed of the air passing it so the mixture screw will have to be turned out a bit or the jet size increased.
height of the fuel tank will also affect the fuel mixture.
if you are putting your finger over the carb to prime it for starting it is probably running out of fuel after a few seconds because the mixture screw is too lean.
the commercial carb with a 6mm bore is meant for a different fuel at much higher revs. Brian likes them but I don't. I am still looking for an easy to build throttled carb design.

 

[Gordon]

The basic design I have been using is from Brian Rupnow and Chuck Fellows. Fairly simple and easy to build.

http://www.mediafire.com/file/cjgmg9kj2vcgy70/RUPNOW-BRITNELL_CARB.zip/file
See: Model I.C. Carb with Throttle

Gordon

 

[coulsea]

The basic design I have been using is from Brian Rupnow and Chuck Fellows. Fairly simple and easy to build.

http://www.mediafire.com/file/cjgmg9kj2vcgy70/RUPNOW-BRITNELL_CARB.zip/file
See: Model I.C. Carb with Throttle

Gordon

Thanks, I have used the fellows design a few times and they work well. I tried the rupnow britnell one once when was a real beginner and made such a mess of it that i gave up, I should try again now that i have more chance of success.
As for your problem, it should work with a bit of tuning, I often take the carb of another engine that i know works to iron out all the other problems that a new engine might have.

Andrew

 

[awake]

Gordon, I can't contribute much ... but when I was working up a Fellows-style carburetor for my Webster build, I found some charts for carb size vs. cylinder displacement - don't recall exactly where - but there was a problem that I didn't realize at first. The chart I saw was for model airplane engines, running at many thousand rpms. It turns out that the expected rpm range of the engine significantly (maybe linearly?) affects the bore size of the carburetor. I don't recall the exact details, but the chart was showing quite large carburetor bore size for my engine - probably in that 6mm range mentioned above - but that was for an engine running at 10K rpm. I wound up making my bore .140", which I'm thinking may still be a bit large for the 1000-3000 rpm range of the Webster, but it has worked pretty well.