What was found
LEVELED AT 4000FT, ENGINE POWER WAS REDUCED TO 80% WITH FUEL CONSUMPTION LEANED TO 17GAL/HR. AFTER BEING SET IN THIS CRUISE PROFILE FOR APPROXIMATELY 2 MINUTES, THE ENGINE BEGAN TO RUN ROUGH. THE PILOT THEN RICHENED THE MIXTURE WHICH DID NOT FIX THE ISSUE, HE PREFORMED MULTIPLE TROUBLE SHOOTING THROTTLE AND MIXTURE ADJUSTMENTS, BUT NONE OF THESE REMEDIED THE ISSUE EITHER PROMPTING THE RETURN TO KDPA. DURING INSPECTION I REMOVED SPARK PLUGS FROM CYLINDER #3 DUE TO CHT READING LOW ON DIAGNOSTIC RUN. VISUALLY INSPECTED AND TESTED TOP AND BOTTOM SPARK PLUGS; FOUND CERAMIC INSULATORS TO BE CRACKED ON BOTH SPARK PLUGS.