PURPOSE: To obtain an optimum air-fuel ratio which is excellent in responsiveness over a whole operation range of an engine and which may directly use an existing carburetor.

CONSTITUTION: An engine 4 incorporates carburetor 21 connected to the intake side of the engine 4, an air cleaner 23 connected to the intake part 14 of the carburetor 21, and an air communication pipe 27 connected to a chamber 25 in the carburetor 21. The air communication pipe 27 may be selectively communicated with the atmospheric air and the intake part 14 by means of a three-way solenoid valve 29. Further, in the case of a two cycle engine which does not use an air cleaner, the air communication pipe 27 is communicated with a crank chamber.