What was found
ACFT DEPARTED FOR LOCAL FLIGHT OVER CITY, ON TAKEOFF EVERYTHING NORMAL, IN CLIMB ENGINE STARTED TO RUN ROUGH. PILOT LEVELED OUT AT 1000 FT AND RETURNED TO AIRPORT. LANDED WITH OUT ANY PROBLEMS. ACFT WAS RUN UP NEXT MORNING STILL RUNNING ROUGH AND MIXTURE WAS VERY RICH. ENGINE SHUTDOWN. ACFT PULLED INTO HANGAR, COWLS REMOVED. IT WAS FOUND ON VISUAL INSP THAT NR 3 EXHAUST VALVE PUSH ROD & SHROUD TUBE BENT. CYLINDER WAS REMOVED. IT WAS NOTED THAT THE EXHAUST VALVE WAS STUCK WIDE OPEN AND IT COULD NOT BE PUSHED BACK IN. LIFTERS CHECKED NO PROBLEMS. NEW CYLINDER WILL BE INSTALLED. CAUSE OF THE STICKY VALVE IS MORE THAN LIKELY FROM LOW POWER SETTINGS IN CRUISE. PILOTS WILL BE TOLD TO RUN ACFT HARDER IN CRUISE.