PURPOSE: To provide high precision controllability for the air-to-fuel ratio over a wide range by furnishing an aux. fuel passage having an opening at a venturi, apart from a fuel passage which includes a measuring jet wherein a measuring needle is inserted in consolidation with a suction piston of this carburettor concerned.

CONSTITUTION: This carburettor concerned includes a suction piston 18, which is situated overstream a throttle valve 14 in the suction air passage 12 in such a way as movable approx. perpendicularly to said passage 12, wherein fuel measurement is performed in a ring-shaped gap formed between a measuring needle 34 fixed to the piston 18 and a measuring jet 40 within a fuel passage 32 formed at the body 10 of the carburettor. Here an aux. fuel passage 42 is provided, which connects a venturi 16 formed by said piston 18 to a float chamber 38. An air bleeder passage 44 of comparative water stem is connected to the horizontal portion of passage 42 extending in high positions, and the air quantity supplied to the aux. fuel passage 42, i.e. the fuel passing this passage 42, is controlled by controlling a solenoid valve 48 installed on the passage 44.