Incident Summary
According to the pilot, during a climb to cruise flight, the engine RPM began to deteriorate. The pilot decided to return to the landing site. Upon arrival, the RPM decayed to flight idle, and an autorotation was initiated. The subsequent landing was hard.
Post-Accident Examination
After the incident, the engine was run for one hour in a test cell and showed no evidence of any malfunction. The fuel speed governor was removed, tested, and found to function normally. However, during the removal of the fuel metering system, a part of an O-ring was found in a spring located next to the fuel ports.
Findings
The O-ring fragment found in the spring could have blocked one of the fuel ports, which would have allowed the RPM to drop to flight idle. No other anomalies were identified during the investigation.