PURPOSE: To prevent an engine torque from being abruptly changed by stopping a feedback control of an air-fuel ratio so as to hold a flow rate control valve of a carburetor at the specified position at a time when a secondary throttle vale is started.

CONSTITUTION: The opening of a primary throttle control valve 4' in NO. 2 barrel 3 of a carburetor 1 is detected by a throttle opening sensor 6 so as to be inputted into a controlling device 21. When the opening of the primary throttle valve 4' exceeds 45°, that is, when the actuation of a secondary throttle valve 5 is detected, the opening of a flow rate control valve 13 which opens and closes an air bleed passages 11 and 12 of the carburetor 1, is held at its position for the specified period of time. With this, an excessive change in the opening of the flow rate control valve is restrained at a time when the secondary throttle valve is started so as to prevent a torque from being changed due to an abrupt change in the air-fuel ratio.