Need a decent fuel filler valve, giant scale
I could be wrong about this but I don't think the diaphragm carbs we use will allow fuel to flow without the engine running. I agree with Kevin and use a third line, except I set it up as a second vent, to empty the tank I just turn the fuse over. It can be filled or emptied from either line. Greg, your not alone, the filler valves I have tried have all failed.