PURPOSE: To improve the fuel consumption and the performance of exhaust gas of the engine without spoiling the re-accelerating performance by a method wherein the throttle valve of a carburettor is closed to a degree less than the opening degree of idling upon the speed reducing operation of the engine in order to prevent the fuel adhered to the inside wall of a suction air path from being swept out by the suction air.

CONSTITUTION: When the throttle valve 4 is closed in order to reduce the speed of the engine, a high vacuum, generated at the downstream side of the throttle valve 4 in accompanied therewith, is taken out of a vacuum take-out port 19 and is introduced into a vacuum chamber 14 of a vacuum operating device 12 through a vacuum path 20, thereby lifting up an operating rod 17. Accordingly, the throttle valve 4 may be closed to the totally closed position thereof after passing the opening degree of the normal idling, therefore, the supply of mixture from the carburettor C to the engine is stopped or decreased remarkably. When the revolving number of the engine is decreased to a predetermined value or less, an engine revolving number detecting switch 23 is opened, therefore, a sloenoid valve 21 shuts the upstream side of the vacuum path 20 because of the cutting of an electric supply therefore and communicates the downstream side of the same with an atmosphere opening port 24, whereby the throttle valve 4 may be opened to the opening degree of idling.