PURPOSE: To improve an exhaust characteristic without causing the deterioration of drivability, by utilizing a feedback signal from an O2 sensor at a point of time when a choke valve exceeds a prescribed opening and performing feedback control of a fuel quantity to obtain logical air-fuel ratio.
CONSTITUTION: A choke valve detector 10 is coupled to a choke valve 3, and this detector 10 is constitutued in such a manner that a reed switch 12 is turned ON when a magnetic path is formed cooperatively by a magnetic path member 13 and a magnetic member 11, mounted to a shaft of the choke valve and rotated. Further an idle switch 15 and full open switch 16, opened and closed in accordance with rotary motion of a throttle valve 5, are provided on a shaft of the throttle valve. Output of each switch 12, 15, 16 is input to a control circuit 20, if output is generated from a NAND circuit 22, an AND circuit 24 is opened to cause an O2 sensor 17 to start feedback control through an output signal. That is, a bleed control valve 9 is controlled by output of the O2 sensor 17 through a driving current waveform generator 25 and amplifier 26.