Abstract: An internal combustion engine includes a duplex carburetor, an engine cylinder, a pair of primary and secondary venturis and an intake manifold connected between the cylinder and the carburetor, the manifold having a common chamber located downstream of the venturis and a pair of primary and secondary branch intake passages opening to the cylinder through a pair of intake ports, respectively. There is a valve in the secondary intake passage for controlling the flow of air-fuel mixture therethrough, the valve being opened by a control mechanism including a vacuum servo which receives a vacuum signal from a port opening to the secondary venturi for actuating link means connected to the control valve. The secondary passage is normally blocked and is only opened when the engine rotates at high speed under heavy load.