What happened
During the cruise phase of flight, the engine RPM decreased from 2,350 to 1,700, causing the aircraft to begin a descent. The pilot performed a forced landing into an open, uneven field. The impact caused the fuselage to buckle and the landing gear to be sheared off. There were no injuries reported in this accident.
The investigation
An examination of the engine following the accident revealed that the number 4 cylinder exhaust valve was jammed in the open position. Investigators found that the valve stem and guide were covered in gummy black deposits.
Records indicated the pilot had been involved in a similar accident on July 22, 1995, where a jammed exhaust valve was caused by excessive carbon deposits resulting from the use of high octane aviation gasoline in a low compression engine.