PURPOSE: To aim at enhancing the performance characteristic of exhaust gas purification, by providing, in a carburettor, a mixture compensating means for adjusting the supply amount of fuel so that an electronic control section controls the air-fuel ratio to compensate the same to a required air-fuel ratio in accordance with the running condition of the engine which given by detection signals from a running condition detecting means and an exhaust gas sensor.

CONSTITUTION: A detection signal generated from a water temperature switch 42 which is turned on and off is delivered to an electronic control section 28 which controls a mixture compensating means 30 disposed in a carburettor 14 so that fuel in a float chamber 54 is fed into a primary intake-air passage 4-3a through a fuel supply passage 28. Thereby, the supply amount of fuel is adjusted to compensate a set air-fuel ratio to a required air-fuel ratio in accordance with the running condition of the engine. Then, the control section 28 controls the mixture compensating means 30 in accordance with a turn-on or -off detection signal from a throttle valve opening switch 40 which judges whether the throttle valve is fully opened or not, to compensate the set air-fuel ratio to be in a transient condition. Further, the set air-fuel ratio is compensated to be in a lean mixture condition and as well super-charged pressure is decreased to conduct the lowering of temperature of intake-air, the purification of exhaust gas, etc.