I replaced my original gas cap for my 70 cutlass recently. The origianl readn ot vented as does the new one. I replaced it due to fuel leaking out around the seal. I know that the gas tank needs to be vented but I think that I need a vented gas cap. I haven't looked to see if my tank isvented, but after driving her for a while I think the pressue builds up and causes it to bog down under heavy excelleration. Can this happen?
When I take the cap off, the WHOOSH is there so it's not being vented. Tomorrow I'l crawl under her to see if there are vent hoses up on top but I know that I have 2 lines going from the tank directly to the mech fuel pump.
Can anyone let me know A. if not properley vented will this cause the engine to bog down. B. If I ahve 2 lines going to the pump, wouldn't I beleive ther eturn line act as a vent?

 

Did you ever figure out anything with the fuel tank vent situation? My fuel tank vent started leaking gas everywhere today, so basically since there was just an open line with the goofy plastic empty vent on the end, I took off the plastic peice and capped off the line. I figured on replacing the gas cap with with a vented one, but did not find one at the parts store.

 

Wow I forgot about that post. Maybe you shouldn't cap off the vent line. The plastic cap should stop the gas from coming out. A new one would probably help out.
As far as the vented gas cap, all of my local auto part stores do not have anything listed for a vented gas cap, but I di find them online on ebay and OPGI or Year One. Like $17 or so.
I found my vent line and think that it's clogged. I'll be working on that soon. I would just open the gas cap every once in a while if I go cruising to release the pressure.

 

I think the '70 cars had vents running up the middle of the back seat in an oval metal tube...'68 and '69 just had two little tubes on the right front of the tank going about 6 inches upward into a clamp...also there is a charcoal canister on your right front fender filler which I belive might be the vent you should check out...

 

My 70 has the two vent lines on the rt. front of the tank. They go into the plastic thingy that is clamped to the trunk drop-off.
The larger line is the vent. The smaller one is to let any fuel that splashes out to flow back in to the tank.