As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ENGINE HAD ERRATIC IDLE AND WAS GETTING WORSE. W.O.T. POWER NORMAL. STARTED ENGINE COLD AND FOUND NR 4 EXHAUST COLD. CHECKED LIFTER DROP CLEARANCE, FOUND IT WAS ZERO AND EXHAUST VALVE SUNK IN SEAT. REMOVED CYLINDER, FOUND BROKEN KEEPERS, WORN KEEPERS GROOVE AND RETAINERS. REMOVED REMAINING CYLINDERS AND FOUND WORN KEEPER GROOVES AND SUNK VALVES AND VALVE SEATING ON OUTER EDGE OF VALVE. (X)