[Power Surge]

If you have an open vent, you're going to get fumes. My car is the same way, and there are always fumes around it when it sits. You can't really block the vent off, because you need a place for air to enter the tank to displace used fuel. If you want to try to cut down on fuel vapors, maybe try installing a carbon tank to run the vent line to. If you are EFI, you can even put a canister purge valve to it also, and pull the fuel vapors back into the intake.