As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PILOT REPORTED THAT THE TACHOMETER WAS INDICATING 100 RPMS HIGHER THEN RECOMMENDED BY THE ENGINE MANUFACTURE AFTER TAKEOFF AND HAD NO CONTROL OF THE PROPELLER GOVERNOR. PILOT REDUCED ENGINE POWER TO KEEP RPMS BELOW REDLINE AND THEN COULD NOT MAINTAIN ALTITUDE. REMOVED GOVERNOR FROM SERVICE AND SENT OFF FOR EVALUATION. THIS EVALUATION DETERMINED THAT THERE WAS INTERNAL OIL LEAKAGE OF OIL PRESSURE CAUSING THE UN-COMMANDED/UN-CONTROLLABLE PROPELLER RPM FLUCTUATION.