PURPOSE: To adjust the quantity of fuel at the time of low and medium speeds simply and securely by adjusting the volume of a bypass chamber provided in a bypass which connects an idle port to a bypass port in a carburetor, by means of a screw.

CONSTITUTION: A bypass chamber 13 is provided in a bypass which connects an idle port 11 to a bypass port 12 in a carburetor, and the end of a screw 18 which is externally rotatable, is projected into the bypass chamber 13. When the quantity of fuel coming from the bypass port 12 deviates from a defined value at the time of inspection after the manufacture, etc. of the carburetor, the volume of the bypass chamber 13 is varied by rotating the screw 18, to adjust the fuel feeding quantity as required, thereby, enabling an adjusting operation easily and accurately.