PURPOSE: To prevent the over-lean of mixture upon accelerating and permit to eliminate suction sound upon a high-speed revolution by a method wherein a vacuum cooperating valve is provided in a suction path at the upstream of a carburettor.

CONSTITUTION: A filter 15 and a suction port 17, inducing atmosphere, are provided in an air cleaner 3 at the upstream side of the carburettor 2. The suction port 17 is provided with the vacuum cooperating type valve 16 while the valve 16 is a variable venturi and is equipped with a reciprocatable piston 19. The vacuum cooperating type valve 16 is chocking the suction path 18 usually, however, when the volume of suction air is increased due to the high-speed revolution of the engine and the vacuum at the downstream side thereof is increased, the vacuum in a vacuum chamber 20 is increased through an orifice 19a and a pressure difference from a pressure chamber 23 is increased thereby opening the suction path 18 of the piston 19. Upon the sudden acceleration, atmosphere enters under opening forcibly the piston 19 of the vacuum cooperating valve 16 against the weight of the piston, therefore, an instantaneous delay is caused and the over-lean of the mixture may be prevented.