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From: Jeffc
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Date: November 23, 2003


Almost all tanks till the late 60s used vented caps, even if you have the 2nd line that runs from the tank to the upper part of the filler tube you still need a vented cap, or a air passage inside the filler tube to let air out when filling the tank, you still need a vented cap......... the only time this is a exception is when the small line runs to the outside air and then it will have a stop built in.... Vented caps are a must for early models because fuel will expand and contract in the tank, causing the tank to compress the fuel in hot weather and have too high of pressure in the system (thus causing leaks at rubber hose fittings on lines or in some cases cause the neddle on the carb to not let fuel in), or cause a vacuum in the tank that will keep fuel from moving at all, often ok when the tank is full, but gets worse as the fuel is used...... If you have a line that runs up to the engine, like later models, that often ends up in the canister or at the carb, then non vented is ok. companies still make vented caps for most standard filler tubes though the books often don't run back into the 50s. If you have a non vented cap and it fits you should be able to cross it to a vented cap...... Vented caps will have a hole in the center with a little piece inside that will close if fuel comes up the filler tube, such as in the case of hard excelleration that send fuel to the back of the tank and up the tube, or a hard turn that will cause the fuel to slosh to one side... If you shake a vented cap you will hear a little ratteling from this piece.


Last changed: May 04, 2010