PURPOSE: To prevent mixture from suddenly becoming excessively rich by providing, in a pilot air passage, a normally-opened type control valve which gradually decreases the passage sectional area while interlocking with engine speed so as to gradually increase the fuel from a pilot system.

CONSTITUTION: A bleed hole in a pilot fuel passage formed in a carburetor body is communicated with a pilot air passage 33, and further on this passage 33 a normally-opened type control valve 35 is arranged. In this case, in the control valve 36 a valve body 36 which partitions the pilot air passage 33 into an upstream side 33a and an downstream side 33b is provided, and further on the circumferential surface of the valve body 36 a communicating hole 37 penetrating in the diametral direction is formed. In addition, to the end part of the valve body 35 an operating wire 40 is connected, and further the operating wire 40 is connected to a wire 13 on the open side of an exhaust valve 11 via a wire connector 39. Thus, when the exhaust valve 11 is opened as the engine speed reaches a prescribed value, it is so arranged that the valve body 35 is moved so that the communicating hole 37 is gradually moved away from the pilot air passage 33.