Gas cap confusion

Thanks everybody, I have the 1965 workshop manual laying around, why I haven’t checked that beforehand I can’t tell you - I’m using it extensively to find some faults in the electrical department so it’s open on those pages. I think it’s because of my confusion (and ADHD fed one-track mind).

The leakages and the surging started on my road trip back home, two weeks ago. I called the previous owner and searched online and found multiple threads on this forum (and Slant Six forum) about the fuel cap being the issue. I could remember that on the last refueling session I turned the cap all the way shut so that’s why it made sense, and loosening it up really helped fixing the problem. The previous owner never had that issue by the way. Anyway, that made me think that the original cap must have been vented normally due to a gap and porous gasket or something.

It now makes much more sense that there must be a clogged vent somewhere else as the tank was leaking badly at the time of leaning out / overheating. The leak seem to come from somewhere at the fill tube side.

I was afraid the tank was leaking because of the vacuum creating leaking seals or even ruptured seams, but after loosely fitting the gas cap the leakage was over too.

So buying an old non vented gas cap will suffice. It will hurt my wallet (there’s one on eBay and shipping costs to the EU are madness) but it’s cheaper than a full tank. And I’m thinking of buying 2 of damraider’s (again via eBay, if his username there is sjd234 - I’d like to support the actual maker of course) so I have one for the Barracuda and one for the Valiant at car shows (locking cap for the daily commutes and parking outside).