On October 3, 2009, about 1515 central daylight time, a single engine Crawford Alan R Glasair II amateur built airplane, N550AC, was substantially damaged during a forced landing following a partial loss of engine power near San Saba, Texas. The pilot, the sole occupant, sustained minor injuries. The airplane was registered to and operated by the pilot. Instrument meteorological conditions prevailed and an instrument flight rules flight plan (IFR) was filed for the 14 Code of Federal Regulations Part 91 personal flight. The cross country flight originated from the Bridgeport Municipal Airport (XBP), Bridgeport, Texas, at 1430, and was en route to the Llano Municipal Airport (AQO), Llano, Texas. According to the pilot, both of the airplane’s main fuel tanks were "topped off" prior to departure. Approximately 45 minutes into the flight and while approaching the AQO initial approach fix (IAF), in instrument conditions and heavy rain, the airplane's engine started to run "extremely rough." Unable to maintain altitude, the pilot declared an emergency and established a descent at best glide speed. Once below the 1,000 foot ceiling the pilot "secured" the engine and performed an emergency landing on rolling terrain among mesquite trees and cacti. The airplane came to rest on its left side with the firewall and engine separated from the fuselage. The pilot was able to exit the airplane unassisted. Following the accident the airplane was recovered to a secure hangar and the engine placed on an engine stand for examination by an airframe and powerplant (A&P) certificated mechanic. According to the airplane's records, the Superior XP-IO-360 four cylinder engine had accumulated approximately 462 hours since new and around 10 hours since its last conditional inspection. The valve covers were removed and continuity was established throughout the engine. Both magnetos produced spark when turned. Compression was established in two of the four cylinders. An examination of the two leaking cylinders revealed tree debris holding the valves in an open position. The fuel screen was found unobstructed. The oil filter was cut opened and no abnormalities were found. The reason for the reported partial loss of engine power was not determined.