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Discussion Starter #1
I've pondered the purpose of the bowl vent on my Autolite 2100 2-V carburetor (C8ZF-G). Problem is after turning off the engine the fuel bowl drains into the intake manifold. I've changed power valve several times using a valve rated between 5-6"HG.
What is the purpose of the bowl vent valve? Could this be contributing to the siphoning of the gas to the intake manifold?
Poor gas mileage less than 12mpg, running rich, and the siphoning are my only complaints.
August 24th, 2011 will be Suntangs 44th birthday and I'd like to celebrate with a trip. Suggestions gladly accepted.
 

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something isnt right other than the power valve. the bowl vent allows the bowl to be at atmospheric pressure which is what makes the carburetor work the way it was designed to. if its overflowing once you shut off the engine my guesses would be

1. that the needle valve or seat is damaged and fuel is leaking past overfilling the bowl.
2. the float is set too high
3. the float isnt floating because it has a hole in it and is full of gas.

not sure what other gaskets being bad could cause this issue.
how long has it been since the carb was totally rebuilt?
 

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Either you are using the wrong power valve gasket or the carb body is cracked allowing fuel to leak.
The vent valve closes when the throttle is closed to keep the fuel that evaporates inside the air cleaner. It is burned the next time the engine is started. You are describing the effects of heat soak - where the fuel in the bowl boils because of heat from the engine and overflows into the intake through the carb venturiis. The cure is a phenolic spacer - high temp plastic that insulates the carb from the engines heat.
Paul
 

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Discussion Starter #4
something isnt right other than the power valve. the bowl vent allows the bowl to be at atmospheric pressure which is what makes the carburetor work the way it was designed to. if its overflowing once you shut off the engine my guesses would be

1. that the needle valve or seat is damaged and fuel is leaking past overfilling the bowl.
2. the float is set too high
3. the float isnt floating because it has a hole in it and is full of gas.

not sure what other gaskets being bad could cause this issue.
how long has it been since the carb was totally rebuilt?
Thanks for the info. I also figured that the needle/seat was the problem. Also classic float setting problems, and bad float.
The carburetor has been rebuilt more than four times in the last 3 years. Completely disasembled with parts cleaned ultrasonically.
This has been going on for a couple of years now. Possibility of a crack housing exists but not probable. Phenolic spacer is in place.
I've been looking for a C8ZF-G at the swap meets without success. Need to find a "decent rebuild" carb. Things are getting tougher to find these days.:bigthumbsup
 

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Discussion Starter #5
Either you are using the wrong power valve gasket or the carb body is cracked allowing fuel to leak.
The vent valve closes when the throttle is closed to keep the fuel that evaporates inside the air cleaner. It is burned the next time the engine is started. You are describing the effects of heat soak - where the fuel in the bowl boils because of heat from the engine and overflows into the intake through the carb venturiis. The cure is a phenolic spacer - high temp plastic that insulates the carb from the engines heat.
Paul
Thanks Paul, I appreciate the feedback. Power valves in various hg increments are hard to find. Walker Products Inc has them however you have to find a dealer to work with. No direct sales. Have replaced at least 5 power valves in the last 24 months, being careful which gasket to use. Original requirements for the power valve (based on operating conditions) is 5.6 in Hg.
Phenolic is in place. If I had a spare carb, I'd take this one and have it magnafluxed.
Thanks again
 

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Don't quote me but I think the power valves are interchangable with Holley????? If so, they should be easier to obtain. However, ven if you have an incorrect one, it should not cause the float bowl to drain down.
 

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Where can I find a vent valve. I am doing a rebuild and the kit doesn't have the bowl vent valve in it. I rebuilt this same carb 20 years ago and that kit didn't have one in it either.
 
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