On May 6, 1997, at 1930 mountain daylight time, a Cessna 172N, N733WJ, registered to and operated by a private owner as a Title 14 CFR Part 91 personal flight, was substantially damaged during a forced landing following a partial loss of power near, Las Cruces, New Mexico. Visual meteorological conditions prevailed, and a flight plan was not filed for the cross country flight. The airline transport rated pilot and his two passengers were not injured. The flight originated from St. Johns, Arizona, about 2 hours and 40 minutes prior to the accident. The pilot reported to the FAA inspector that while in cruise flight the engine RPM decreased to 2,100. He pulled the throttle out and the RPM decreased to 1,900 RPM. He then applied full throttle and the RPM only increased to 2,000. The pilot performed a forced landing to a young pecan orchard and the airplane nosed over. Examination of the aircraft by the FAA inspector revealed that both wings sustained structural damage, and the nose landing gear and right main landing gear were separated from the fuselage. Examination of the aircraft's fuel system revealed that the right fuel tank contained approximately 20 gallons of fuel, and the left fuel tank was empty. Further examination of the aircraft's throttle assembly revealed that the throttle arm castellated nut was loose, and the cotter pin was missing allowing the throttle arm to move without actuating the throttle shaft. A review of the maintenance records revealed the throttle shaft was replaced 6.2 hours earlier during an annual inspection. The person who performed the throttle shaft replacement reported that the airplane left his facility with the cotter pin installed. Repeated attempts to obtain a completed Pilot/Operator Accident Report, NTSB Form 6120.1/2, were unsuccessful.