Abstract: An automatic priming system for internal combustion engines, which is operable to prime the carburetor of the engine at engine cranking speeds, and is automatically disabled after the engine starts. The automatic priming system is driven by pressure fluctuations within the engine crankcase which are caused by reciprocation of the piston. At engine cranking speeds, fluid communication of positive and negative pressure pulses between the engine crankcase and an accumulator is allowed via a restrictor. The positive and negative pressure pulses are used to drive an air pump, such as a diaphragm air pump, to pump atmospheric air to the carburetor to pressurize the fuel bowl of the carburetor for priming. After the engine starts and the piston reciprocates more rapidly, the restrictor prevents communication of positive and negative pressure pulses between the crankcase and the accumulator, and the pressure within the accumulator is relatively constant and below atmospheric pressure.