THE PILOTS WERE CONDUCTING AN OBSERVATION FLIGHT OVER THE NORTHEAST END OF THE SAN FRANCISCO BAY WHEN THE AIRPLANE'S ENGINE FAILED. THE PILOTS WERE UNABLE TO RESTART THE ENGINE AND TURNED TOWARDS A ROADWAY NORTH OF THE BAY TO LAND. THE AIRPLANE LANDED SHORT OF THE SELECTED LANDING AREA AND STRUCK A FENCE AND A UTILITY POLE. AN EXAMINATION OF THE FUEL INJECTOR SERVO AFTER THE ACCIDENT DISCLOSED THAT THE FUEL CONTROL DIAPHRAGM STEM FAILED INTERRUPTING THE FUEL SUPPLY TO THE ENGINE.