On March 12, 1997, at 1625 hours Pacific standard time, a Columbia XJ-7, N48RW, collided with soft terrain and nosed over while attempting an off airport forced landing at Camp Pendelton, California. The aircraft sustained substantial damage; however, the pilot and mechanic were not injured. The aircraft was being operated as a maintenance test flight by R. W. Martin, Inc., when the accident occurred. The flight originated in Carlsbad, California, at 1605. Visual meteorological conditions prevailed and no flight plan was filed. The pilot stated that he experienced a partial engine failure about 20 minutes after takeoff. He notified McClellan-Palomar tower that he was making a forced landing near Las Pulgas Road, west of Interstate 5. After touching down in an open grassy area, the hull dug into rain-softened terrain and the aircraft nosed over onto its back. The accident site was located within a restricted area of Camp Pendelton Marine Base at 33 degrees 18 minutes north latitude by 117 degrees 28 minutes west longitude. This was the aircraft's first flight after an annual safety condition inspection. An inspection of the aircraft by FAA inspectors revealed throttle movement in the cockpit did not produce corresponding movement of the butterfly valve in the carburetor throat. The throttle linkage did not show evidence of impact damage. The operator reported that the carburetor is designed to automatically go to 80 percent power in the event there is a mechanical separation in the throttle linkage. A review of the aircraft logbooks did not reveal any discrepancies. A final attempt by the operator to diagnose the cause of the engine failure was unsuccessful.