PURPOSE: To improve accelerating properties by a method wherein the vacuum of a suction pipe is increased upon sudden acceleration to eliminate the waste of fuel in the carburettor equipped with the accelerating device.

CONSTITUTION: An annular tube 10 is arranged at the outer periphery of a main nozzle 7 and an annular clearance 11 is formed by the outer periphery of the main nozzle 7 and the inner periphery of the annular tube 10. The annular clearance 11 is connected with an air path 13 from an air pump 12, operated during an engine is being operated only, while the air path 13 is provided with a normally closed type opening and closing valve 14 opening the air path 13 for a given time interval upon the sudden opening of a throttle valve 4 only. Upon the sudden acceleration, pressurized air is injected from the annular clearance 11 and the tip end vacuum of the main nozzle 7 is composed with a vacuum due to the pressurized air in addition to the vacuum of a venturi, therefore, the vacuum may be increased. According to this operation, the amount of fuel sucked out of the main nozzle 7 may be increased and mixture, sufficiently rich and suitable for the acceleration, may be supplied.