Incident
Approximately 45 minutes after takeoff, the pilot detected a burning odor and subsequently noted a loss of engine power. The engine did not cease operation entirely but reduced to approximately 1000 RPM. The oil pressure dropped to about 10 psi. The pilot attempted to restore power by jockeying the throttle and applying carburetor heat, but neither action had any effect. The aircraft then executed a forced landing, during which it encountered a ditch and nosed over.
Engine Examination
A teardown of the engine revealed that the lifters for the #3 and #4 intake valves were chipped, spalled, and worn. The corresponding cam lobes were rounded and worn. The engine was reassembled with the original parts and subsequently operated normally. The oil pump, oil filter, and oil temperature and pressure sensors were checked and found to be normal. No mechanical reason was identified for the loss of engine power.
Environmental Conditions
At the time of the incident, the temperature was 76°F and the dew point was 64°F. According to icing probability curves, serious icing could occur at glide power, and visible icing could occur at rated power.
Findings
The pilot reported a burning odor and power loss. The engine teardown showed worn valve lifters and cam lobes, but these components did not prevent normal operation after reassembly. No mechanical cause for the power loss was determined.