PURPOSE: To improve exhaust gas properties and running properties, by a method wherein, in a carburetter which control an air-fuel ratio by feedback by means of an output from the O2 sensor of an exhaust system, when the output from the O2 sensor outputs an abnormal lean signal, a power valve is actuated.

CONSTITUTION: According to an output from an O2 sensor 19 in an exhaust pipe 17, a control device 20 controls an air-fuel ratio by feedback through control of air bleed control valves 14 and 15 of a carburetter 1. When an output from the O2 sensor 19 outputs an abnormal leans signal, the control device 20 opens a valve 30 in an open air release passage 29 of an introduction pipe 26 for an intake air pressure exerted on a piston for actuating a power valve 22 of the carburetter, and actuates a power piston to open the power valve 22. This constitution enriches air-fuel mixture and improves an output. When the O2 sensor 19, however, outputs a lean signal, it is decided that the control device is abnormal.