On May 25, 2001, at 1520 hours Pacific daylight time, a Cessna 150H, N23474, sustained substantial damage when it nosed over during a forced landing following loss of engine power due to fuel exhaustion near Mariposa, California. The private certificated pilot, the sole occupant, was not injured. The airplane was being operated by the owner under the provisions of 14 CFR Part 91. The personal cross-country flight departed Whiteman Airport, Los Angeles, California, at 1315, and was destined for Pine Mountain Lake airport at Groveland, California. Visual meteorological conditions prevailed, and no flight plan was filed. In his report to the Safety Board, the pilot stated that the engine stopped after 2.1 hours of flight, when the airplane was about 16 miles from the destination and with the fuel gauges indicating 1/8 full. He attributed the fuel exhaustion to an interruption that occurred during his (self) fueling of the aircraft, which resulted in the fuel tanks not being totally full on takeoff. The pilot reported that, as he was filling the left wing fuel tank before takeoff, he was interrupted by another pilot who asked his assistance with operation of the self-fueling equipment. He completed filling the left tank and stopped to assist the other pilot. He returned to his airplane about 10 or 15 minutes later, filled the right-wing tank, and departed. In his report, he stated he believed that, during the interruption, a portion of the fuel in the left tank flowed across the aircraft to the right tank and, after he filled the right tank, the left tank was approximately 5 gallons below capacity. The aircraft had been modified to install a larger engine than the original and had a 24-gallon usable fuel capacity. The pilot reported making the trip between Whiteman and Pine Mountain Lake regularly. Assuming a zero to 5-knot headwind, the trip routinely required 20.3 or 20.4 gallons to complete. He reported that, on past flights at the same altitude and power setting, the engine consumed 9.17 gallons per hour (2.6 hours endurance).