PURPOSE: To securely inject accelerated fuel with excellent responsiveness, by providing a check valve in the negative pressure passage of the negative pressure type acceleration pump of a carburetor for permitting the pump to store negative pressure in it and providing a valve for releasing the negative pressure in the pump to the atmosphere when a throttle valve opens.

CONSTITUTION: The fuel chamber 37 of an acceleration pump 34 is disposed in the lower portion of the float chamber 4 of a carburetor 1 so that the chamber 37 communicates with the float chamber 4 and an injection nozzle 33. A spring 39 is disposed in a chamber 38 on the back of a diaphragm 36 separating the fuel chamber 37 and the chamber 38 serves as a negative pressure chamber 38 connected to a suction passage 2 downstream of a throttle valve 7 through a check valve 71. The negative pressure chamber 38 communicates with the atmosphere through a passage W1a, a valve 55 and a passage W1b. The check valve 71 permits negative pressure sucked while an engine is in operation to be kept at its maximum pressure in the negative chamber at all times. When a lever 68 turns in the case that the throttle valve 7 is released, a valve 55 is opened to cause the chamber 38 to communicate with the atmosphere. As a result, the negative pressure in the chamber 38 is lost, whereby accelerated fuel is injected by the spring 39.