[andyh1956]

That check valve looks like a serious fuel restriction to me!
If some yall think the fuel in the carb is draining back to the tank, please explain how the fuel in the carb bowl can push the float & needle up, get up & over into the seat inlet openings, down past two check valves in the fuel pump & then up & over into the tank.
Maybe the fuel could heat & expand enough to raise the float up.
It would be a simple test to take the inlet line loose from the pump & set a receptacle under it after the initial flood of gasoline & then watch it.
Q jets don't hold very much fuel in the bowls to begin with & any leak drains them fast.
I'v had lots trouble with diesel engines having fuel drain back but never a gasoline engine.