PURPOSE: To improve a performance of engine in its medium and high speed ranges by a method wherein a plurality of venturi parts are provided in the primary intake passage.

CONSTITUTION: Fuel in the float chamber 1 passes through the primary main jet 6 and enters the primary main well 7. The fuel is mixed with the air suctioned from the primary main air jet 8, injected into the inner venturi 10 of the primary venturi from the primary main nozzle 9 and then supplied in the primary inlet passage 2. Signal from the O2 sensor 14 is processed by an electronic control device 13 and a solenoid valve 12 is operated by its output signal. The lower part of the float chamber 1 is communicated with a secondary main jet 15, further communicated with a starting mixture jet 23 and a starting air passage 22 through the starting fuel jet 20 and with the starrting air jet 21. Further, when a high speed operation is performed, a degree of opening of the primary throttle valve 4 becomes high, the secondary throttle valve 5 is opened in response thereto, the fuel from the secondary jet 15 and the air from the secondary jet 17 are mixed in response to a flow rate of air flowing into the secondary intake passage 3 and then supplied to the secondary inner venturi 19.