Abstract: An internal combustion spark ignition piston engine has a main combustion chamber and an auxiliary combustion chamber connected by a torch opening. A main carburetor supplies a lean mixture to the main combustion chamber and an auxiliary carburetor supplies a rich mixture to the auxiliary combustion chamber. A fixed orifice is provided in the auxiliary carburetor, instead of a movable throttle valve, the orifice being smaller than the torch opening but large enough to allow air flow at subsonic velocities when the engine is operated under idle conditions. A throttle valve in the main carburetor controls the speed of the engine but the fixed orifice limits the amount of rich mixture supplied to the engine. A fuel compensator system is provided for each of the carburetors to control the air-fuel ratio of the mixture produced by each, in response to engine operating conditions, including idling, high power and deceleration.