PURPOSE: To make the whole carburetor compact and prevent the flow of surplus fuel by making a fuel measuring member which is arranged at the entrance of a constant oil surface chamber interlocking with an air valve in the air valve type carburetor which makes the negative pressure of a mixture chamber almost constant.

CONSTITUTION: The air valve 2 and the throttle valve 3 are provided in the carburetor 1 and form the mixture chamber 4 between both valves 2 and 3. The constant oil surface chamber 5 has the fuel inlet 16 introduced through the fuel tank 12, pump 13, and filter 14, the fuel outlet 6 which supplies fuel to the fuel injection port 7 opening at the mixture chamber 4, and the overflown port 25 which returns fuel to the tank 12. The fuel measuring members 19 and 20 are movably arranged at the fuel entrances 6 and 16. These members 19 and 20 engage with the axis 9 of the air valve 2 through the levers 21 and 22 and are ON/OFF controlled simultaneously by the air valve 2.