PURPOSE: To enable to construct a maintenance-free carburetor comprising a throttle valve controlling means consisting of a motor unit and a control circuit, by employing such an arrangement that the throttle valve controlling means is rendered inoperative during operation for warming up the engine but set into operation when warm-up of the engine is completed.

CONSTITUTION: When an engine is started, negative pressure at a downstream part of a throttle valve 6 is transmitted via a vacuum passage 12 into a vacuum chamber of a choke piston 10 and lowers the pressure in the chamber, so that a choke valve 4 is opened. At the same time, current is supplied to a heater in a choke-valve chamber , so that the opening of the choke valve 4 is increased by a bimetal. On the other hand, a wax heat-sensitive member 17 is also heated, so that the opening of a throttle valve 6 is decreased gradually through displacement of a rod 21. Subsequently, when the engine operation shifts to idling operation as the engine is warm- ed up by the automatic choke mechanism and throttle valve controlling mechanism, a control circuir 35 of the throttle valve controlling mechanism produces an output and actuates a motor unit 26, so that the opening of the throttle valve 6 is controlled at a proper angle and therefore a stable idling operation can be obtained.