Feel like I'm chasing Cotton around the board, but here goes: IF the vent isn't blocked (and I've had that *exact* same problem on my tanks, a little bit of work with a pair of sissors on the gasket fixed that), then tweaking the seal in a different way may help. First, test the fit with a clean, dry seal on the cap. If you've got an original or good repro tank with a 1/4 twist "bayonette" style cap, it should be really hard to get it to fully close, i.e. all the way to the stop. If it's not, then *gently* bend the tabs on the cap in or out until you have to really twist to get it to close, and it stops shy of the stop tab. In other words, when you tighten the cap, it should really be clamping the seal tight when it's closed. If the tank's ramps are worn or bent (the inverse ramps that the cap tabs ride on when you're tightening the cap), you're never going to get a good seal. Consider replacing the whole gas cap insert (you can buy weld in replacements from a lot of sources).
All that said, the problem is the hot motor heating up the gas in the tanks and causing expansion, pushing the gas out of the tank. It *can* be made worse by blocked vent in the cap, or a restricted cross over tube, but it also *will* happen if you fill your tank with cold gas from an underground tank then let it cook on a hot motor. Or air temp gas on a hot motor, for that matter. Remember two things: 1) the left tank's gas level will always be higher when it's on the side stand; and 2) the right tank will get more heat from the motor 'cause it's more directly on top of the hot engine. So it heats, expands, and pushs gas out of whichever tank has the loosest seal. Your only defense is to not fill the tank fully when you know you're going to park it right afterwards, and seal the tanks as best you can using techniques described above. HD knew this, that's why they had center stands...
-Kuda
'49 panchop
