Vented gas cap?

[Snakebit]

Bob, Is the gas cap vented on the FIA? I am running Webers with a return line. Pressure builds up in the tank and after a day or so, fuel begins to drip out the throttle shafts on to the manifold. I have to open the gas cap to release the pressure to prevent this from happening. What gas cap do I need?

[strictlypersonl]

I did a little research today and the present cap is a Stant 10810 (fits 1980 Mustang w/3.3L). I don't have a number for an equivalent two-way cap, but RockAuto lists the specs, which might help matching something up.
The problem is exacerbated by weak float resistance - Webers are only meant to run with 2.5 PSI.

[Snakebit]

Just went to Advanced Auto Parts and matched up a #10640 Stant vented cap, which fits a 1964 Ford Falcon or a 1955 Chevy all pre 1970. I'll see if this helps. Thanks.

[jeffy]

Bob-

Is this something I should get for ERA 2131? Maybe John can order one.

Thanks,

Jeff

[smallblockhead]

Originally Posted by jeffy

Bob-

Is this something I should get for ERA 2131? Maybe John can order one.

Thanks,

Jeff

I think a vented cap is a necessity for a car with a carburetor. How do I know? My vented cap got plugged up and it built up enough pressure in the tank to push fuel past the needle valve in a Holley double pumper, flooding the engine routinely when it was shut off during a warm autocross event last year. Getting it started again was a bit dicey, and was definitely not environmentally friendly! Once I figured out the issue and cleared the vent, everything was fine.

Gary

[WardL]

Question, I have an overflow line to the ground inside the fender. Hmm, is that outside or inside the tank/cap? I've never heard the gas venting when I've undone my gas cap.

[FOMOCONUT]

WardL ; The overfill vent only works when the cap off, its above the cap seal. SPF #2773 Jerry

[strictlypersonl]

For safety reasons, I'd rather stick with the one-way vent cap unless someone has a problem. This thread is the first I've heard of fuel leakage. Considering that fuel pumps have a check valve that keeps 5+ psi in the system all the time, I'm a little suspicious of problems occurring with 4bbl-equipped cars. In order to get 5 psi in the tank, the (absolute) temperature in the tank would have to rise by more than 100 deg F.

[Snakebit]

Vented caps only vent in, not out. So fuel pressure was building up inside the tank from the return line. I took the gas cap and drilled a 1/16 hole through the vent. Now it vents both ways and no more fuel leaking on the manifold.