Could be a couple of things, when you shut down the motor the fuel will siphon back down through the pump. You could also have some slightly leaky bowl plugs if you're running a Q-jet. The Q-jets also had different types of primary jets, some had a small hole in the side allowing fuel levels to drop when shut off, others did'nt have that hole and the fuel level was maintained longer. In any event the bowl is vented to the atomosphere and all will evaporate over time. Some longer than others, mine is good for 3 days max without any starting problems. After that she cranks for about 8-10 seconds.