PURPOSE: To improve connection between primary and secondary lines, by forming an accelerating well communicated with an accelerating pump in a secondary intake line of the duplex carburetor and discharging fuel from the well as a secondary throttle valve is released.

CONSTITUTION: A secondary intake line 2' involves a low-speed system composed of a float chamber 21, main jet 3, step jet 5, fuel line 7 and injection port 8 around a secondary throttle valve 2. An accelerating well 9 is set through a jet 12 to a fuel line 7 downstream an air jet 6 to feed fuel by means of an accelerating pump 10 while the engine is being operated. A fuel return line 14 communicated with the float chamber 21 is set below the fuel well 11 to return fuel from the well 11 to the float chamber 21. Fuel in the well 11 is discharged at the moment the secondary throttle valve 2 is released to improve the relationship.