PURPOSE: To prevent the occurrence of percolation by forcedly opening a power valve when a cooling water temperature is not less than a prescribed tempera ture and an air conditioner is turned on and an engine is in idle operation in a carburetor having a power system to supply fuel through the power valve so as to increase a quantity.

CONSTITUTION: When an engine is operated, in an ECU 20, first of all, whether or not an air conditioner is turned on is judged on the basis of an air-conditioner on/off signal, and when the air conditioner is turned on, whether or not a cooling water temperature is not less than a prescribed temperature, for example, 95°C is judged. When a judgment here is Yes, next, whether or not it is in idle operation is judged on the basis of an engine rotating speed signal and a throttle opening signal, and When it is in idle operation, when feedback control under which opening of an air bleed control valve is not less than 90 steps continues for 60 seconds or more, a vacuum directional control valve 19 is opened to atmospheric pressure, and a vacuum chamber 15 is put under atmospheric pressure, and a power piston 16 is pushed down by spring force, and a power valve 11 is opened.