Might not apply here, but my H2B had similar fuel issues, not as bad as that, i had bought a repro fuel cap, and my bike would fall off at higher speeds, longer pulls, the new cap had poor air flow, punched holes that had burrs in the way, a gasket that the hole didn't line up with the cast piece hole, etc. Try dismantling the cap, can do it on the tank still, and deburr and check all labrinth air flows are clear. Made a decent improvement on my bike.