PURPOSE: To reduce the capacity of a catalyst when the fuel consumption is reduced compared with a conventional case and when a catalyst for purification of exhaust is to be provided at the time of making the air fuel ratio of a mixture generated by fuel supply from a carburetor constantly smaller than a theoretical air fuel ratio so as to improve the driving performance of a vehicle.

CONSTITUTION: The air fuel ratio of a mixture 24 generated by fuel 12 supply of a carburetor 3 in relation to intake air 23 is made to be constantly smaller than a theoretical air fuel ratio. An exhaust detection sensor 33 for detecting the condition of exhaust 25 is provided. The carburetor 3 is controlled by the detection signal from the exhaust detection sensor 33 to adjust the amount of the fuel 12 supply.