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 for controlling communication between an amount of fuel increasing passage which is connected to each air intake of a multiple carburetor and which has an amount of fuel increasing pump interposed therein, and a return passage.

CONSTITUTION: A throttle valve 9 to be manually controlled is slidably provided within the sliding valve guide tube 3 of the main body 1 of each carburetor A1 to A4 provided to respective cylinders. 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, etc., An amount of fuel increasing passage 21 through which an increased amount of fuel is supplied from an amount of fuel increasing pump 20 to each control jet 71A to 71D disposed opposite to the air intake 2 of each carburetor A1 to A4 is provided and a control valve 23 which opens the amount of fuel increasing passage 21 and blocks communication between the passage 21 and a return passage 22 when the throttle valve 9 is open for more than a predetermined degree and the rotating speed of an engine is within a fixed range is interposed in the passage 21.