PURPOSE: To improve a degree of accelerability as well, by interconnecting a suction pressure chamber for opening a suction piston of a variable choke carburetor to a suction pipe via a solenoid on-off valve, while largely opening the valve in time of deceleration and also larely opening the suction piston, eliminating its fuel suctional force and reducing an extent of a valve opening at the latter half period of deceleration.

CONSTITUTION: A chamber 11 of a spring 13 of a suction piston 3 at the upstream side of a throttle valve 5 is provided with a Venturi suction pressure hole 14, while it is interconnected to a suction pipe via a pipe 22 and a solenoid valve 23. In time of engine deceleration, a decelerating state is detected by each output of a suction pressure detector 24 and a rotation sensor 49, and at first, an opening duty ratio in the solenoid valve 23 is largely taken as specified whereby the piston 3 is largely opened with an extensive intake of suction pressure, making a Venturi part turn to atmospheric pressure, whereby suctional force for fuel is brought to nothing and thus a fuel cut takes place. According to the elapse of time, the duty ratio of the solenoid valve is reduced slowly so much at engine cooling time, while a supply of fuel is gradually increased, thus responsiveness in time of successesive acceleration is also improved.