Sounds to me like you've got a problem in the carb, not in the supply system. Like Mike & Charlie said, the engine starts from the fuel in the bowl, and then the bowl is refilled quickly by the pump as the engine cranks & starts. Something is letting your fuel drain from the bowl. Assuming the casting is good I'd wonder about a high float level getting a siphon started through the jets, and that draining your bowl. If you go out and look into the carb throat after it's been sitting for a few hours you might see fuel dripping into the intake from the accelerator pump nozzles or from the primary/secondary nozzle in the venturi? Worth a look.