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Allis Chalmers Discussion Forum

B floods/runs rich

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Gary MacR

11-09-2011 18:09:48

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I am working on my father-in-laws B with a Zenith carb that he said sort of backfired and then started missing. I took the carb apart and cleaned everything out, put it together and had the float set too high so gas ran out of the vent on the bottom. I adjusted the float so it seals fine but it still runs way too rich. If I turn the fuel shut off just about all the way off it runs like a watch.

I have been working on engines for many years, so I think I have all the obvious stuff covered, but this is driving me nuts. I have adjust the idle jet and even shutting it off makes no difference. This carb does not have a power jet/high speed jet.

Any suggestions greatly appreciated.

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Dick L

11-10-2011 03:53:30

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 Re: B floods/runs rich in reply to Gary MacR, 11-09-2011 18:09:48  
The power jet does have to have a little hard red washer type gasket under it or you (could) get gas thru the threads as well as thru the orifice. The float on that carburetor is not all that critical. Turn the top upside down and make the float parallel both sides.

third party image

third party image

Set the idle screw at idle only. After you are above idle you can remove the screw and you will have no change because the engine is above idle.
The idle screw is an air mixture screw for fuel mixture at idle to get the proper mixture thru the idle circuit. The power jet limits the fuel to mix with the air drawn thru the vacuum venturi to get the proper mixture. Without a power jet mixture adjustment screw the proper mixture can only be achived by having the correct amout of air drawn thru the venturi by the engine. To get a leaner mixture you would need a smaller power jet or more vacuum at the venturi. You could have a slight leak at the carburetor around the throttle lever shaft or the carburetor gasket. An engine with low compression could also cause a lower vacuum at the venturi.

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Dick L

11-12-2011 07:05:28

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 Re: Passing Along Some Observations in reply to Dick L, 11-10-2011 03:53:30  
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These venturi are from Zenith carburetors that I have apart rebuilding at this time. They are all four from non adjustable power jet carburetors and as you can see they have different size openings. The smaller ones came from 1938 B's, the larger ones came from C's. The carburetors look the same in every way except for the venturi and the orifice of the power jet. Mix the parts up when assembling or get them on the wrong engine and it will not give the best running tractor. Up grading the older engine with larger pistons will also change the carburetor engine balance without upgrading the carburetors internal parts.

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11-09-2011 19:35:44

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 Re: B floods/runs rich in reply to Gary MacR, 11-09-2011 18:09:48  
First off set the idle adjustment to about 1.5 turns out and leave it alone it only works at a closed throttle idle. Double check you have the float set as it should be and maybe even a little closer to the lean side. You say he said it back fired so you also need to check the air cleaner and make sure it is not clogged up and or full of dirt/water. Also and ya this may sound funny check spark it could be on the weak side and or the points may need to be adjusted.

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