It leaks out every time you accelerate!I believe the aftermarket tank neck is not at the proper angle allowing gas to go back against the cap.Tom

 

Tom - why wouldnt you be able to use the drilled cap all the time (even when driving)?

 

View attachment 69 Firebird Gas Cap.pdf Well, I found a new gasket and rather than removing and replacing, I decided to install over the the top of the original because the new gasket was half the thickness of the original.

 

My 69 has a aftermarket tank and it does not have the factory vent in the filler neck.I have 2 caps I use,one for driving which is a vented cap that leaves air into the tank.The other is for when the car is parked as I drilled a small hole in it.If yours is a factory tank you should have no issue finding the correct vented cap.The issue I had with the vented cap it let air in but would not let it out with a non vented tank when the car set in direct sun and the tank would gain pressure from the expanding gasses and it forced fuel past the neddle and seat and filled a couple cyc with gas and locked up the engine.I was lucky and figured it out as the engine would not crank enough to light as it would have cracked the block.Lot of words to tell you to research what you have.Tom

 

Well , since I didn't catch any replies in my trolling for info I decided to remover the rubber grommet and I'm off to the hardware store to grab some new materials. I'll let ya know how I make out...