The aircraft struck a fence during a forced landing, following a loss of engine power in flight. The pilot reported that he had topped off both main fuel tanks prior to departure. About 10 minutes after departure at about 4000 ft MSL, he smelled fuel fumes in the cockpit. He observed that the fuel level gauge for the tank in use was indicating 1/2 full. The engine completely lost power shortly thereafter. Efforts to restart the engine by turning on the boost pump and switching tanks were unsuccessful. Post-crash examination of the aircraft by the FAA revealed evidence of a fuel gascolator leak. The gascolator gaskets appeared to be homemade. No problems were found to exist in the rest of the fuel system or the engine. The aircraft had acquired 2 hours flight time since the last annual, which was accomplished 3 weeks before the accident. The mechanic who performed the annual reported that he had disassembled and cleaned the gascolator prior to completing the inspection.