What was found
(CAN) DURING FLIGHT TO PRACTICE AREA, ACFT DEVELOPED A VIBRATION AND LOST RPM. CREW DECIDED TO RETURN TO AIRPORT. RUN UP ON GROUND PERFORMED AND CONFIRMED ROUGH RUNNING ENGINE. TROUBLESHOOTING DISCOVERED THAT EXHAUST VALVE ON NR 3 CYLINDER WAS STUCK IN THE FULL OPEN POSITION. NO OTHER ABNORMALITIES NOTED. CYLINDER REMOVED, VALVE STEM AND GUIDE CLEANED OF BUILD UP. VALVE RE-INSTALLED AFTER CLEANING. CYLINDER RE-INSTALLED. GROUND RUN COMPLIED WITH AND ENGINE RAN WITHIN NORMAL OPERATING PARAMETERS. ACFT WILL BE RETURNED TO OPERATIONAL STATUS ONCE A MX CHECK FLIGHT IS COMPLIED WITH.