 Often when a rider notices the motor start to sputter, he or she can access the reserve before the engine shuts down.In most cases, the main and the reserve settings are actually drawing from the same tank, but there are two outlets through which the fuel may leave.One outlet is located a short distance above the other, when the fuel selector is set to the 'main' position, the fuel will flow from the upper outlet, and will stop flowing when the fuel level gets below the outlet.