As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
JUST AFTER TAKEOFF, THE LEFT ENGINE BEGAN TO RUN ROUGH. THE AIRCRAFT RETURNED FOR LANDING AND A COMPRESSION CHECK WAS PERFORMED. THE NR 6 CYLINDER HAD LOW COMPRESSION DUE TO LEAKAGE PAST THE EXHAUST VALVE. THE EXHAUST VALVE APPEARED NORMAL EXCEPT FOR DISCOLORIZATION FROM HEAT.