Abstract: An internal combustion piston engine drives a rotary supercharger of the positive displacement substantially continuous flow type, the supercharger turning in proportion to the engine speed. A power control valve is positioned upstream from the supercharger for controlling the flow of air-fuel mixture to said supercharger. Objectionable fluctuations in speed of the engine when idling are minimized by a secondary valve operatively positioned between the supercharger and a valved intake passage leading to the combustion chamber of the engine. The operation of the secondary valve and the power control valve are coordinated so that the secondary valve remains in idle position when the engine is idling but remains in open position during all other engine operating conditions.