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Quote:
Originally Posted by David Cockey
Bill, you're correct. Your notes caused me to do a little research. According to Steve Pargeter's excellent monograph on Model A carburetors an "bowl vent hole" was added in May 1930. Before that there was no obvious vent between the bowl and atmosphere. Perhaps this holds the key to why some Model A's can have vapor lock problems.
For a carburetor to work properly the air/vapor above the fuel in the bowl needs to be at atmospheric pressure. In normal operation with the float valve system working properly and the fuel not boiling the fuel level in the bowl doesn't vary much. This results in the volume of air above the fuel in the bowl not varying much. Not much airflow is needed to keep the air at atmospheric pressure so the "vent" doesn't have to be very large. In a Model T Holley carburetor the vent hole is small and under a brass tag. In the earlier Model A carburetors leakage of air past the gasket must usually suffice given the number of Model A carburetors without bowl vent holes which work quite nicely.
However, if the fuel is boiling in the bowl and the only vent is the small amount of leakage past the gasket then problems are likely. Fuel may slow or stop flowing into the bowl. If the vapors don't vent quickly enough the engine will eventually stop running as the fuel level in the bowl drops.
With the bowl vent hole the vapor will vent to the atmosphere.
Perhaps the solution to vapor lock in a Model A is to add the bowl vent hole to the carburetor if it's missing. As note above Paul Moller recommended this.
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Actually there is a bowl vent in all the Model A carbs I've seen, as shown in the picture above. A small secondary vent was added in mid 1930.
The vent is toward the rear of the 3 holes shown in line about the middle of the top of the carb bottom. The vent hole was enlarged during 1929.