PURPOSE: To prevent adhesion of fuel to a wall surface by providing a small venturi downstream of the initial opening position of an air valve in a secondary side of a carburetor and further providing a fuel nozzle there.

CONSTITUTION: An air valve 4 provided in a secondary side of a carburetor is opened by air flow generated by the opening of a throttle valve 1 which opens in association with a throttle valve 2 in a primary side. In response thereto, the opening area of a fuel jet is defined through a lever 6 and a rod 8. There is provided downstream of the initial opening position of the air valve 4 a small venturi 14 which is eccentrically displaced from the center of a bore and has a main nozzle 11 extending therethrough and includes a mouth ring 15 so as to form a clearance passage therearound. When the air valve 4 is partly opened, most of air flow passes only the small venturi 14 to ensure atomization of fuel. Thus, there remains no fear of adhesion of fuel on the wall surface of the bore.