PURPOSE: To improve maneuverability and air fuel ratio property in acceleration by providing an jet of an accelerating nozzle in an annular space defined by large and small Venturi tubes in a carbureter provided with an accelerating pump mechanism.

CONSTITUTION: A jet 17 of an accelerating nozzle 16 is provided in a large negative pressure position of an annular space defined by the outer wall of a small Venturi tube 4 and the inner wall of a large Venturi tube 3 to be directed downward toward a throttle valve 2. When the valve 2 is opened for acceleration, an accelerating pump piston 11 interlocked with the valve 2 pressurizes fuel, pushes up an outlet check ball 13 and introduces metered fuel through an accelerating jet 14 to the accelerating nozzle 16 so that the fuel is injected from the jet 17 into an admission pipe to maintain proper air fuel ratio in the acceleration. Also, an amount of air passing through the large Venturi throat is increased by opening the valve 2 at this time, and fuel reserved in an accelerating fuel reservoir 19 is injected from the jet 17 together with air introduced from an air leak jet 15 to improve further the responsive property of air fuel ratio in the acceleration.