Incident Overview
An instructor reported that after completing a series of stalls, the engine began to backfire continuously and lost power, preventing the aircraft from maintaining altitude. A forced landing was made off the airport, during which the aircraft struck a fence during the landing roll.
Engine and Maintenance History
The engine had been overhauled and installed in the aircraft 36 hours prior to the incident. A Federal Aviation Administration (FAA) maintenance inspector described the overhaul as good. No engine maintenance had been performed since the engine was installed. The aircraft was parked on an outside ramp.
Post-Inspection Findings
An FAA inspector found that the nuts and lockwashers were missing from the No. 3 cylinder valve rocker shaft hold-down plates. This allowed the exhaust rocker shaft to back out of the cylinder and the intake rocker shaft to back out of the cylinder. As a result, the No. 3 cylinder lost operation, leading to a significant loss of engine power. All remaining rocker shaft nuts were tight.
Conclusion
The investigation identified the missing hardware on the No. 3 cylinder rocker shaft hold-down plates as the cause of the power loss. No probable cause was officially stated in the source material.