On January 2, 2000, at 1430 hours Pacific standard time, a Cessna 172A, N7249T, experienced a partial loss of engine power and came to rest inverted after making an emergency landing in an open dirt field near Dos Palos, California. The airplane, owned and operated by the student pilot under 14 CFR Part 91, sustained substantial damage. The pilot received minor injuries. Visual meteorological conditions existed for the instructional flight that had departed the Merced, California, airport at 1400. No flight plan was filed. In an interview with a Federal Aviation Administration (FAA) inspector, the student pilot reported that she had refueled the airplane several weeks prior to the accident flight. The airplane was refueled with approximately 30 gallons, and she had not flown it since it had been refueled. The pilot stated that during the preflight she verified that she had "in excess of 15 gallons" of fuel in each wing by utilizing a graduated fuel gage. She then checked for fuel contamination by taking a sample from the port of each wing with no discrepancies noted. She reported that there were no problems encountered during the run-up or the departure from Merced. While en route to the practice area, she performed several practice turns. About 30 minutes into the flight, she noted a loss of altitude and added throttle to regain altitude, but did not receive a corresponding power indication. She noted the fuel selector was in the both position and attempted to troubleshoot the problem by switching to the left and then right fuel tank positions, and applying carburetor heat. She stated that there was no change and the rpm's continued to drop. The engine instruments were crosschecked and she reapplied carburetor heat with no change. The pilot reported that the engine was not running rough or sputtering, it was just not producing enough power to sustain flight. During the landing rollout, the nose gear dug into the plowed dirt field, collapsed, and the airplane nosed over and came to rest inverted. The student pilot indicated that she had purchased the airplane in June 1999, without the logbooks. The Airframe and Powerplant (A & P) mechanic who performed the annual inspection of the airplane sent a letter to the former owner requesting the logbooks. The former owner responded by indicating that the logbooks had been accidentally burned with the trash. In the last entry for the annual performed on June 6, 1999, the A & P mechanic indicates that the carburetor was replaced to comply with Airworthiness Directives 72-06-05, 93-19-04 and 98-01-06. An engine inspection was conducted at Plain Parts in Pleasant Grove, California, on February 9, 2000. It was noted that the retaining nut that holds in place the serrated throttle lever to the serrated stop of the throttle shaft of the carburetor did not have a cotter pin. The retaining nut was found loose on the shaft, allowing the serrated portion of the throttle lever to not engage properly with the serrated stop of the throttle shaft.