As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PILOT REPORTED ROUGH RUNNING ENGINE AFTER START UP. INVESTIGATION BY MAINTENANCE DETERMINED THAT THE #3 CYLINDER EXHAUST VALVE SEAT/VALVE HAD DROPPED OUT OF CYLINDER HEAD. LYCOMING FACTORY OVERHAULED ENGINE WITH 637.5 TSOH, 1646.2 TTSN.