On May 6, 2000, at 0800 mountain daylight time, a Noteman RV6-A homebuilt experimental airplane, N456TP, was substantially damaged during a forced landing following a partial loss of engine power while in cruise flight near Kanab, Utah. The non-instrument rated private pilot was not injured and his passenger sustained minor injuries. The airplane was owned and operated by the pilot. Visual meteorological conditions prevailed for the Title 14 CFR Part 91 personal flight. No flight plan was filed for the 195 nautical mile cross-country flight from Hurricane, Utah, to the Canyonlands Field Airport, near Moab, Utah. The flight originated from the Grassy Meadows Sky Ranch (UT47), near Hurricane, Utah, approximately 0730. The 683-hour pilot reported that while in cruise flight at 8,000 feet msl (2,000 feet agl), the engine started to run rough after he heard a "pop" sound come from the engine compartment. The pilot added that the airplane was not able to maintain altitude, and he elected to perform an emergency landing to an estimated 1,800-foot stretch of a paved road. During the approach to the road, the pilot observed a vehicle entering the road in the opposite direction, and the pilot elected to abandon the approach and land on an 800-foot long meadow adjacent to the road. The pilot stated that the airplane landed hard, the nose wheel sank into the soft ground, and the airplane nosed over to the inverted position. There was no fire and both occupants exited the aircraft unassisted. Examination of the 4-cylinder engine by the pilot/builder of the airplane revealed a broken exhaust valve. The pilot/builder stated, in the enclosed Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1/2), that the valve failure was "probably caused by old intake gasket/intake air leak." The engine had accumulated a total of 1,600 hours.