As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING PRE-TAKEOFF RUNUP, ENGINE STARTED VIBRATING AT MID-RPM RANGE AND WOULD NOT ACCELERATE. FOUND NR 3 AND NR 4 EXHAUST VALVES STICKING. REAMED GUIDES TO PROPER DIMENSION. ENGINE HAS 107.6 HOURS SINCE FACTORY OVERHAUL. THIS OPERATOR HAS HAD NUMEROUS LOW TIME ENGINES WITH THIS PROBLEM. AIRCRAFT HAVE DASH 10 FUEL INJECTORS AND RUN ON 15W50 OIL AND ALSO RECEIVE PROPER COOL DOWN.