PURPOSE: To simplify a control program and supply proper air-fuel mixture, by disposing fuel pressure regulators in main and auxiliary passages to a negative pressure type carburetor and disposing a temperature sensing valve and an idle increasing electromagnetic valve in a throttle valve bypass passage.

CONSTITUTION: A carburetor having a piston 103 to be actuated by the negative pressure portion of a diffuser 104 is closed when it starts because a small negative pressure of the diffuser portion is applied to fuel pressure regulators 112, 113 and starting fuel shut-off valve 129 is opened because an air intake pipe 101 has also a small negative pressure. A jet 131 to be opened and closed by an electromagnetic valve 132 subjected to a duty proportional control by a main jet 109 and a control unit 145 supplies fuel into a bypass air passage 122 bypassing a throttle valve 102. The passage 122 supplies proper air-fuel mixture under control of a cooling water sensing valve 123 and an idle increasing electromagnetic valve 148. The valve 129 is closed and the valves 112, 113 are opened after the start.