PURPOSE: To avoid air/fuel ratio from being made lean caused by the delay of feedback control, so as to prevent engine speed from stalling by changing the opening of an air bleed control valve of a carburetor to a proper second opening smaller than a first specific opening.
CONSTITUTION: If an engine speed when the throttle valve 12 of a carburetor 10 is opened and moved from an idle opening is less than the set engine speed, the opening of an air bleed control valve 22 is once changed to a second specific opening. The air/fuel ratio is feedback-controlled by changing the opening step by step by a fixed opening on the basis of a detection signal of an exhaust sensor 14 after the abovementioned change so as to reach the expected value. Simultaneously the opening of the air bleed control valve 22 is once changed to the first specific opening larger than that of the second specific opening when the engine speed is more than the set engine speed. A control means 26 is provided for feedback-controlling openings by changing the specific opening step by step by a fixed opening on the basis of the detection signal of an exhaust sensor 14 so that the air/fuel ratio may reach the expected value. Thereby, air/fuel ratio can be prevented from bearing rich.