A flow controlling method for an auxiliary intake flow passage for use in a power unit which includes a starter clutch which is automatically engaged and disengaged in response to the engine speed. An intake air control valve (IACV) carries out changeover from water temperature dependent control (open loop control of adjusting the flow rate of the auxiliary intake flow passage in response to the warming up state of an engine) to target engine speed feedback control when the actual engine speed NE [rpm] drops to a clutch disengagement engine speed NEout (engine speed at which the centrifugal clutch is disengaged) (S3: Yes) and the opening θ1 [degree] of the throttle valve becomes smaller than a predetermined threshold value (throttle valve opening threshold value TH—θ1 (S4: Yes).