PURPOSE: To promote the improvement of startability in the time of low temperature, by controlling an opening and closing valve in an acceleration fuel return passage in accordance with a condition of a manual choke valve opening and closing and an engine temperature.

CONSTITUTION: A carburetor 1 provides in its choke valve 3 a manually opening and closing mechanism 24 and a choke valve opening and closing detecting switch 22 which operates being associated with the mechanism 24. A return passage 16 of an acceleration fuel passage 9 provides an actuator valve 17, and its control circuit mounts a temperature detecting switch 21 and choke valve opening and closing detecting switch 22 to be interposed. In this way, when the choke valve is closed in the time of low temperature starting, startability can be enhanced by opening the actuator valve 17 to prevent a mixture from its over rich state.