67 vented gas cap

[Daniel G.]

My gas cap on my 67 427/435 says "vented" but it only lets the air move into the tank and seems to have a diaphram that doesn't allow air to de-pressurize the tank. When I remove the cap to add fuel it blows air out of the tank and almost jumps out of my hand. I seem to remember in the old days guys would simply drill a 1/8 inch hole in their gas caps to vent them. Shouldn't the venting work both ways?
Daniel Grunwald

[John H.]

Re: 67 vented gas cap

Shouldn't the venting work both ways?

Yes, but only when it exceeds a certain pressure - that's why there's a spring in the vent valve; if the cap rattles when you shake it, it's probably OK.

[Stuart F.]

Re: 67 vented gas cap

The earlier cap, circa 63, had just a single vent hole. Often times these were enlarged to improve venting (so thought). The better cap though is the two way valve cap like your's should have. I replaced my original 63 cap, first with a vented locking cap (which I removed when I found you could open it with a nickle), then with the later two way vented cap which, by the way, is still available from GM. It works fine and is not all that expensive. If you continue to have concerns for yours, perhaps you should replace it.

Stu Fox

[Clem Z.]

Re: 67 vented gas cap

this excess pressure in the fuel tank with the 3X2 carb setup caused a lot of engine problems because it would allow fuel into the oil pan as the needle and seats could not seal because the end carbs have a shorter fulcrum arm to clear the metering plate

[Daniel G.]

Re: 67 vented gas cap

Thanks for the replies. I think I'll try to check for a rattle first. Maybe a drop kick across the driveway will work when it blows out of my hand next time. If it doesn't rattle suficiently I'll replace it.
Daniel

[Bill I.]

Re: 67 vented gas cap

Dan, don't drop kick it, you may miss and throw out your knee. Suggest just throwing it like a disc. It should rattle. Bill.