As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PILOT REPORTED ENGINE RPM WENT FROM 2500 DOWN TO 1700 IN CRUISE FLIGHT. ENGINE CONTINUED TO OPERATE SMOOTHLY AND PRODUCED POWER. PILOT RETURNED TO THE AIRPORT UNDER POWER AND MADE A NORMAL LANDING. VISUAL INSPECTION APPEARED NORMAL BUT DURING THE STATIC ENGINE RUN UP THE ENGINE PRODUCED 26" OF MANIFOLD PRESSURE BUT ONLY 1400 RPM. AFTER REPLACING THE PROPELLER GOVERNOR THE AIRCRAFT OPERATED NORMALLY.