On July 19, 1996, at 1034 hours Pacific daylight time, a Robinson R22B helicopter, N730SH, rolled over on landing from an autorotation near Morgan Hill, California. According to the pilot, the autorotation was precipitated by an engine and main rotor overspeed situation on the engine-rotor tach which the pilot could not resolve. The helicopter was operated by Nice Air of San Jose, California, and rented by the student pilot for a local area solo instructional flight. Visual meteorological conditions prevailed and included calm winds. No flight plan was filed for the operation. The helicopter sustained substantial damage. The student pilot, the sole occupant, was not injured. The flight originated from the Reid-Hillview Airport, San Jose, on the morning of the accident and had made an en route stop at the South County Airport, San Martin, California. The pilot reported in his written statement that the helicopter was in cruise flight at 1,000 feet agl when he noticed that both the engine and main rotor were at 110 percent on the dual tachometer. He then entered an autorotation because he believed the engine might quit. The student stated that at touchdown the helicopter had a nose low attitude with some forward speed remaining and the skids dug into the ground, rolling the helicopter over. Through a translator, the pilot reported that the engine/main rotor governor was not working properly. Robinson Helicopter Company reported that the governor unit senses main rotor speed through a sending unit on the engine to transmission shaft. An electronic computer maintains the speed within the normal operating range through a servo motor which actuates the throttle on the collective by means of a clutch assembly. The unit can be turned off by a toggle switch on the collective control, and can be overridden by twisting the throttle. The entire governor assembly, including the speed sensing unit, computer, servo, clutch, and collective/throttle were removed from the helicopter and taken to Robinson Helicopter Company by a Safety Board investigator. All components of the unit were functionally tested and performed within factory new unit acceptance criteria.