Join Date: Nov 1999
Location: Central California, USA
You need a valve that can be free flowing going into the tank, and yet hold up to seven pounds of gas fumes in.
Seven pounds is what I recall is the Reid vapor pressure. That's the pressure that gas fumes will make when put in a container. Above 7 lbs gas will not vaporize or produce more fumes.
The factory gas caps hold pressure like this. If the tank were overfilled, then the LIQUID gas would expand with more pressure than that so there needs to be a way to vent liquid fuel or your tank would burst, or the carb would be overpressured and soak the engine.
On my race car I just have an aircraft check valve, a flapper type. It has a light spring on it. That allows air in when running and no fuel out at all. I put it inside the tank cover so it's sitting in fuel when full.
I had a friend who had just a hose there, no valve and when he put a funnel in the fuel filler and poured fuel in, the fumes came pouring out of the vent!
A mechanic told him he didn't need a check valve in it. I had him stand there by the vent while I poured the fuel and he gagged!
This was on a LOLA coupe and the vent went into the engine compartment. One spark after fueling up and POOF! A $175K car would be toast. All because of a missing $20 check valve.
I had another case where I was racing a guy at a Laguna Seca vintage event. He had a vent hose running up high on his roll bar. When he accelerated hard, fuel would spray out of the vent. I was right behind him and got sprayed. It looked like oil smoke but it was fuel! Scary!
I'll check some of my catalogs and see if there is anything. You might also check with the fuel cell mfr.
The older I get, the faster I was!