[AROWHED]

I battled this problem for a long time. On my 65, the factory fuel tank vent line system (just like 66bonne described it) always spilled gas under hard acceleration, or if tank was above half full, or if was a hot day, etc. So I tried closing that vent line off and using a vented gas cap. They did not fit tight enough and also spilled fuel. So I went back to a non-vented cap and tried extending the length of the factory vent line...still no luck.

Final solution (so far) was to use tight fitting, non-vented cap, closed off factory fuel tank vent line, dropped gas tank and added an entirely new vent line from the fuel level sending unit assembly, which then runs up into the area above rear axle so it is higher than fuel tank, then follows frame to jsut short of rear bumper where it is then open to the atmosphere.

I recently spllced a large metal fuel filter into this vent line, mounted vertically, to filter any incoming air, but more importantly it acts as an "exchange point" for gasoline vapors to bypass any liquid fuel trying to escape up the vent line. So far it seems to have worked, but I try not to fill the tank above half...and can't afford to anyway!