PURPOSE: To prevent mixture from becoming lean when an engine is operated at low speed with a throttle valve wide open by providing an amount of fuel increasing pump within an amount of fuel increasing passage which supplies fuel from inside a fuel tank to the air intake of a carburetor, and providing the amount of fuel increasing passage with a control valve for controlling communication between the amount of fuel increasing passage and a return passage.

CONSTITUTION: A throttle valve 9 to be manually controlled is slidably provided within the sliding valve guide tube 3 of a carburetor main body 1 through which an air intake 2 is bored. A needle 13 for controlling the area of a passage of a jet 14 communicated with a float chamber 5 is mounted to the throttle valve 9. Fuel is supplied from a fuel pump P to the float chamber 5 via a fuel inflow passage 17 and a valve seat 6. An amount of fuel increasing passage 21 for supplying fuel increased by an amount of fuel increasing pump 20 to a control jet 34 disposed opposite to the air intake 2 is provided and a control passage 23 which opens the amount of fuel increasing passage 21 and shuts out communication between the passage 21 and a return passage 22 when the throttle valve 9 is open for more than a fixed degree and the rotating speed of an engine is within a fixed range is interposed in the passage 21.