On your next cold start, remove the air cleaner and hit the carb with a shot of starting fluid. If it starts immediately that tells you that you're losing fuel somewhere, probably draining back out of the carb, or boiling out after shutdown as carbking suggested. A transparent fuel filter near the carb also will show you if there's any fuel in the line when you crank.
BTW, you don't by any chance have a Pertronix points-replacement ignition in that, do you? Their older models used to have that idiosyncrasy.
