So I'm out riding today, stop to fill up. No more full than ever before. Get on bike, start up and gas starts running out from under cap. Shut off quickly and get off. Clean up mess. As I hop on bike, I hear a hissing noise and gas starts running out from under cap again. Every time I bounce the bike a little or jostle it, I hear hissss and then leak. Finally quits doing it and I ride away wondering, WTF ?? Any idea's? Do I need a new gas cap?

IDK if I just got lucky, or if thisa actually helps, but after its gassed up & before I put the cap on,, I wiggle it a little,, I also spent some time putting a thin plastic shim under the rubber gasket so it closes just a hair tighter, Have a look at the sealing edge of the tank. Is it smooth>

OK here is my history of my battle with the gas cap leak:
1) replaced original gasket with the spud washer, which promptly turned punky.
2) Packed out original washer with supposedly gas proof gasket material I had laying around- still leaked.
3) taped up filler hole in tank and smoothed and polished the contact area of the neck with emery cloth. Still leaked but not as much.
4) installed the Honda gasket- leaked worse.
5) Cut the hole in the Honda gasket larger so it would fit over the edge of the lock assembly and seat right against the cap and re-installed the original gasket on top of it. This worked fine for a while, but the Honda gasket shifted, blocking the vent intake, and causing the bike to act like it was running out of gas. It would start up after a few minutes and run for a few minutes before it would starve for gas again.
6) Cut a slot in the Honda gasket and positioned the slot so as not to block the vent, and I haven't had a leak since.