a portable pneumatic tool includes a valve closure member interposed in the tool motor exhaust passage and engaged with a speed responsive mechanism comprising a plurality of flyweights operable to move the closure member to an open position on starting of the tool motor . on decreasing motor speed a coil spring assisted by a fluid pressure drop across the closure member forces the closure member to move against the decreasing centrifugal force exerted by the flyweights to shut off motor exhaust fluid flow to stop the motor . the closure member is normally held in a partially open position by a second spring to enable the motor to be started . an alternate embodiment includes a limited motion coupling between the valve closure member and the motor drive shaft which provides assistance in opening and closing the valve closure member .