PURPOSE: To improve the stability of an engine by eliminating air fuel feedback control when the machine is cool and using signals such as neutral and clutch switches to distinguish running from stop state, and by controlling a flow control valve to control an air fuel ratio.

CONSTITUTION: In an ECU 16 which inputs output signals from an O2 sensor 8, a water temperature sensor 18, a neutral switch 20, a clutch switch 22, a crank angle sensor 24, an idle switch 26 and an ignition coil 28, when the machine is warm, outputs from the O2 sensor 8 are used to perform feedback control to remain the air fuel ratio unchanged. On the other hand, when it is determined that the machine is cool, feedback control is eliminated and running or stop state are determined based on signals front the neutral switch 20 and the clutch switch 22. When it is determined that the machine is running, a flow control valve 14 provided in an air bleed channel in the carbureter 2 is opened and when it is determined that the machine is stopped, the flow control valve 14 is closed to hold the air fuel ratio to the 'rich' side.