PURPOSE: To promote the improvement of the response characteristic and the improvement of fuel consumption, by simultaneously controlling the air amount of an air bleed and the fuel amount of a control jet in a carburetor by a single valve and deciding the fixed speed running from the change rate of an engine speed performing a lean burn control.

CONSTITUTION: A carburetor 1 provides a fuel passage 15, bypassing between a float chamber 5 and the side of a main air bleed 8 in a main fuel passage 6, and a control jet 16 halfway the passage 15. While the carburetor 1, in which an air passage 17 communicates with the downstream side of a slow jet 12 in a slow fuel passage 10, provides a control air bleed 18 halfway the passage 17. A valve unit 16a of the control jet 16 is connected to a valve unit 18a of the control air bleed 18 by a common rod 19, and one of these valve units is opened when the other is closed. While a lean burn control is performed by detecting the change rate of an engine speed to decide the time of fixed speed running. In this way, an engine enables the response characteristic of air-fuel ratio control to be enhanced further the fuel consumption to be reduced.