PURPOSE: To improve accelerating response at a time of low temperature, by providing a device to make a fuel passage area variable in accordance with temperature, downstream from a fuel passage for the accelerating pump of a carburetor.

CONSTITUTION: The accelerating pump 5 of a carburetor, when a lever 9 is in swing operation, ejects the fuel in a chamber 6 from a nozzle 3 through a non-return valve 8 and the holes of a valve plate 19 into intake air. The valve plate 19 is reversed at high temperature, and lets the fuel flow only through a central hole 17, thus supplying accelerating fuel properly at a time when temperature is high. When temperature becomes low, the valve plate 19 is restored, and in addition to the center hole 17, lets the fuel flow through notches 18 located around the central hole, thus diminishing flow resistance in a passage. The fuel is supplied also through an auxiliary passage 11 by opening a solenoid valve 12 by the operation of a heat-sensing switch 15 as occasion demands at a time when temperature is low.