Depends on the engine really - some B&S motors have an internal governor, obvious because of a spring loaded lever that sticks out of the crankcase. Others have an air governor, which can be seen if the cooling cowl is removed from the engine. Governor problems are fairly rare (especially the internal types). Chances are there is a spring that has come off. With the engine off, the governor should hold the throttle open. With the engine running, the throttle should close against the throttle stop screw on tickover.
John, July 2008