On September 4, 2001, at 2030 hours Pacific daylight time, a Cessna 150J, N51256, experienced a loss of engine power and made a forced landing in a vineyard approximately 2 miles northwest of the Sonoma Skypark Airport, Sonoma, California. The airplane was operated by the student pilot/owner under the provisions of 14 CFR Part 91, and sustained substantial damage. The pilot, the sole occupant, was not injured. Visual meteorological conditions prevailed for the night cross-country flight, and no flight plan had been filed. The airplane had departed the Calaveras County-Maury Rasmussen Field Airport, San Andreas, California, about 1930, and was scheduled to terminate at the Sonoma Skypark Airport. The Safety Board investigator interviewed the pilot. The student pilot stated that he had refueled at Sonoma Skypark the morning of September 3. He flew to the Calaveras County airport and spent the next 2 days working in San Andreas. The pilot stated that there were no mechanical anomalies noted with the flight to Calaveras County. On the night of September 4, he conducted a preflight; however, he did not refuel the airplane prior to leaving Calaveras County. The pilot indicated that he had flight following during his flight back to Sonoma, but that he had not filed a flight plan. During the return flight he was in radio contact with the Sacramento and Stockton air traffic controllers, and Oakland Center. While in contact with Oakland Center, he requested an altitude change near Sonoma from 4,500 feet to 3,000 feet. He stated that he was over the airport, but could not see the airport. As the airplane reached an altitude of 3,500 feet, the engine lost power. The pilot reported he made several unsuccessful attempts to restart the engine. He declared an emergency with Oakland Center. The pilot stated that Oakland Center attempted to give him directions to the airport; however, the airplane was losing altitude and he decided to land the airplane. He picked a road to land on; at the last minute he saw power lines and maneuvered to avoid them. The airplane came to rest on its nose in a vineyard. The pilot stated that the engine was sputtering and then quit. During the attempts to restart the engine he stated that the primer was in and locked. He further stated that he did not see the oil pressure gage indicating anything abnormal. A Sonoma County Sheriff's deputy who responded to the accident site reported that fuel was present in the fuel tanks, but he could not tell what the quantity was. A Federal Aviation Administration (FAA) inspector, who conducted the on-scene inspection, stated that 3 1/2 gallons of fuel were drained from the left tank and 1 gallon of fuel was drained from the right tank. He further indicated that a few drops of fuel were collected from the fuel bowl. The inspector conducted an engine inspection and ground-run. During the engine inspection he noted that the ignition switch was defective. The inspector interviewed the pilot, who was present for the engine inspection. The pilot stated that there was a "trick" to starting the airplane. Pressure had to be applied to the instrument panel in order to allow the ignition switch to positively ground to the system for start up. The pilot further stated to the inspector that he knew the switch was defective prior to the accident flight, and it had been that way for some time. No further discrepancies were noted with the engine ground run. According to the airplane manufacturer, the fuel system is a gravity fed system. Total fuel on board the airplane was 26 gallons; usable fuel for all flight conditions was 22.5 gallons, with 3.5 gallons unusable fuel. The closest aviation weather reporting station was the Napa, California, airport, which is located about 6 miles east of the accident site. The 2054 METAR report for Napa noted in part that the temperature was 60 degrees Fahrenheit and the dew point was 55 degrees. Review of a carburetor icing probability chart disclosed that this temperature and dew point falls in an area of the graph annotated "Serious Icing Climb or Cruise Power."