Gas cap frustration

Jan 3, 2006
This thing has me stumped. It's a Briggs 130232-0169 5 HP on an old Black & Decker generator. It was resurrected and everything works, but... The gas cap leaks. The cap is not bent, the old gasket is in decent shape, and the tank mouth is smooth. I have tried making new gaskets using gasket paper and rubberized cork, and there's no difference. Even if the tank is only 1/3 full, if I move the generator, it sloshes and copious amounts of gas ooze out from around the gas cap (not the air vents). Even while it's running, gas oozes and trickles out. When I shut it down, it usually shakes coming to a stop, and it gets wet around the cap.

Now here's the weird thing. If I take the gas cap off while it's running, it stays dry! I can see gas is sloshing around in the tank from vibration, and the internal float bowl is trickling over and back into the tank as it's supposed to, so there's stuff happening, but only the occasional drop splashes out if the cap is off. I can confirm that if I make a fresh gasket, there's a nice even groove in the gasket where the tank mouth seals. If I make a rubberized cork gasket with no holes, I can rock it back and forth and there's no fuel coming out. I can only figure it's either soaking into the gasket paper like a sponge and oozing out between the threads. It seems to be sealing against the inside of the cap just fine.
A piece of a Scotts Shop Towel (blue) taped around the tank mouth should tell you where the gas is leaking from. Solution can be a piece of plastic food wrap around tank neck with hole in it with cap on or maybe dielectric grease on gas cap threads.