PURPOSE: To improve the suction capacity of a carburetor, such as an Amal type carburetor or the like, having a throttle piston moving across an air passage over the whole operating range thererof by forming the air passage bore to have an upper half on the full admission side with a larger section than that of a lower half on the full closure side.

CONSTITUTION: In a carburetor which is provided with a throttle piston 3 that moves across an air passage 2 of the carburetor body 1, the air passage 2 is so formed that the area S of the lower half (with a height 1/2 H) of the passage section (with a height H) on the full closure side is smaller than the area S' of the upper half on the full admission side. This increases an intake air flow rate on the closure side of the throttle piston to accelerate a fuel vaporization, securing an ample amount of the intake air when the throttle piston 3 is largely opened.