On March 17, 2007, at 1800 mountain daylight time, a Mooney M20B, N74503, experienced a partial loss of engine power after takeoff and forced landed near St. George Municipal Airport, St. George, Utah. The private pilot operated the airplane under the provisions of 14 CFR Part 91. The pilot and single passenger were not injured, and the airplane sustained substantial damage. Visual meteorological conditions prevailed, and no flight plan had been filed. The flight originated at St. George airport at 1745, and was en route to Blackfoot, Idaho. The pilot stated to the National Transportation Safety Board investigator that 10 minutes after takeoff he leveled the airplane at 6,500 feet for cruise; the engine started running rough and backfiring. The engine would run sporadically rough then smooth out. The pilot executed emergency procedures, switched fuel tanks, and switched on the boost pump, but there was no change in the engine's rough running condition. He decided to return to St. George airport, and configured the airplane for best glide. He setup for a 2-mile final to runway 34. The pilot determined that he was not going to make the runway, and force landed the airplane in a clear area by a river bed. The next day a Federal Aviation Administration inspector examined the engine and determined that the right magneto contained engine oil. The seal between the engine accessory drive and the magneto was hard, loose fitting, and covered with engine oil. Examination of the engine maintenance logbook revealed that the engine had been manufactured new on February 8, 1979, and installed on the airplane April 13, 1979. A 9-year gap where no maintenance was recorded on the engine occurred between October 1992 (engine total time of 1302.9 hours) and July 2002 (engine total time of 1322.39 hours). A 100-hour inspection was recorded as being completed on July 2, 2002. The most recent 100-hour inspection was performed on June 22, 2006, at 1,439.1 hours total time. There is no record of an engine or magneto overhaul. Lycoming Service Instruction No. 1009AS states that engines that do not accumulate the hourly period of time between overhauls specified (2,000 hours for the O-360-A1D) are recommended to be overhauled in the twelfth year. Teledyne Continental Ignition Systems Service Bulletin, SB643B, for all TCM and Bendix magnetos states that the magnetos must be overhauled or replaced at the expiration of 5 years since the date of original manufacture or last overhaul, or 4 years since the date the magneto was placed in service, which ever occurs first with out regard to accumulated operating hours.