Thanks for everyones input.

The only cap I've been able to find that will fit this thing is made by Stant, locking or not. Neither it turns out are vented. Although looking at the inside, theres a piece in there that has 5 little holes in it and derned if it don't look like some kind of venting set up. EPA strikes again. Cured my problem though. Buddy of mine has an old Packard Roadster and I went and looked at his cap. Small hole drilled in the lock down ring and thru the spring plate. Doesnt go all the way thru the cap. Had to remove that inner plastic piece with the fake vent holes though, problem solved.

I would like to know, if anyone can tell me, what is causing this tank to build up pressure? Is it all due to heat? Exhaust does run right along side it. But it happens in short time.

I would think that as much as that carb drinks, it would create a vacuum.