PURPOSE: To prevent mixture from becoming lean when an engine is operated at low speed with a throttle valve wide open by controlling according to the operating condition of the engine a control valve which controls communication between a fuel inflow passage leading to a carburetor and a return passage, the fuel inflow passage branching off from the fuel inflow passage to supply fuel to an air intake.

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 from a fuel pump p is supplied to the float chamber 5 via a fuel inflow passage 17 and a valve seat 6. An amount of fuel increasing passage 20 which branches off from the fuel inflow passage 17 to supply an increased amount of fuel to a control jet 33 disposed opposite to the air intake 2 is provided and a control valve 22 which opens the amount of fuel increasing passage 20 and shuts out communication between the passage 20 and a return passage 21 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 20.