On March 21, 1997, at 0900 hours mountain standard time, a Cessna 150L, N21993, collided with trees during a forced landing 9 miles northwest of Nogales, Arizona. The forced landing was precipitated by a loss of engine power during cruise. The aircraft was owned and operated by the pilot. Visual meteorological conditions prevailed at the time and no flight plan was filed. The aircraft sustained substantial damage. The certificated commercial pilot, and passenger were not injured. The flight originated from Ryan Field, Tucson, Arizona, at 0830 as a personal cross-country flight to Nogales. In the pilot's written statement, he said that at the time of the partial engine power loss they were flying at 7,500 msl over the Tucumcari Mountains. The power dropped, then returned to cruise rpm, at which point the pilot changed his heading to get away from the mountains. About 30 to 40 seconds later the engine lost power again, regained power, and then dropped again. This cycle was repeated seven or eight times, with the pilot performing the emergency checklist procedures. The engine then stopped completely and the pilot made a forced landing where the aircraft collided with trees. The aircraft retrieval firm which recovered the wreckage reported that approximately 10 gallons of a yellowish colored fuel were found in each tank. An engine run-up was conducted by a Federal Aviation Administration (FAA) inspector from the Flight Standards District Office, Scottsdale, Arizona, with no mechanical abnormalities found. The FAA inspector stated that a fuel sample was retrieved from the aircraft fuel tanks at the accident scene and appeared to be contaminated. As viewed, the sample had some water and debris in it, and was yellow in color. Particles of rust were also noted in the sample. The inspector reviewed the maintenance records and found that the owner had obtained an STC to use auto fuel in this aircraft. He reported that "the sample smelled like a mixture of auto fuel and aviation fuel. . .[and]. . .neither of these fuels are yellow in color." The aircraft owner/pilot stated to the inspector that he has a barrel of fuel at his hangar from which he refuels the aircraft.