PURPOSE: To cancel spewing phenomenon and to prevent enrichment of air fuel ratio by providing a solenoid valve to open and close an opening communicated from a float chamber of a carburetor to each of inner and outer air vents and a solenoid valve to insert and remove a metering needle and a rod to cut off fuel.

CONSTITUTION: A first solenoid valve 13 has a first slide valve 14 and a second slide valve 15, and it opens and closes openings from a float chamber 40 to inner air vents 6, 7 and outer air vents 8, that is, a primary inner air vent passage 9, a secondary air vent passage 10 and an outer vent first passage 11. Additionally, a second solenoid valve 19 of two-step motion type opens and closes the openings by way of inserting and removing a metering needle 20 and a rod 21 to cut off fuel into and out of a main jet 5 and adjusts discharge of fuel from a main nozzle to a carburetor venturi part. Consequently, each of the solenoid valves 13, 19 is controlled by ECU 23 on the basis of signals from a fuel temperature sensor 24 and others provided in fuel inside the float chamber 40.