On August 25, 2003, at 1335 Pacific daylight time, a Piper PA-18-150, N2567L, veered off the runway and nosed over during a precautionary landing at a private airstrip 13 miles southeast of Avenal, California. The forced landing was precipitated by a partial engine power loss. The airplane was operated by the private pilot/owner under the provisions of 14 CFR Part 91. The pilot and one passenger were not injured. The airplane was substantially damaged. Visual meteorological conditions prevailed, and no flight plan had been filed. The personal flight originated at a private airstrip 2 miles north of Lost Hills Airport, California, about 1330. The pilot stated to the National Transportation Safety Board investigator that 5 minutes after departure, and at an altitude of 500 feet, he heard "a loud noise" from the engine with an ensuing reduction in power. By manipulating the carburetor heat control he regained some power. He landed at the first available landing site, which was a small private airstrip. During the landing roll the airplane veered right, and the pilot corrected to the left. The airplane departed the runway and into soft ground on the left side of the airstrip. The airplane nosed over onto its back. The pilot said the airstrip appeared to have been freshly oiled down. Meteorological data recorded at Bakersfield Airport, Bakersfield, California, at the approximate time of the accident show that the temperature was 95 degrees Fahrenheit with a dew point of 60 degrees Fahrenheit. A standard carburetor icing chart indicated that at those temperature and dew point levels, light carburetor icing was likely when in the glide or cruise power configurations. Under the supervision of a Federal Aviation Administration inspector, an Airframe & Powerplant mechanic examined the engine and found no anomalies. The mechanic was then able to start the engine and achieved 800-900 rpm before shutting the engine down due to vibrations caused by the bent propeller.