What was found
WHILE CRUISING AT 7500 FLYING VFR FROM KRNT TO KHWD, 3H20MIN INTO THE FLIGHT, I LOST MY CYLINDER #4 (LYCOMING IO-390). THE ENGINE SOUND SUDDENLY CHANGED AND IT STARTED VIBRATING MILDLY. DECLARED A MAYDAY WITH OAKLAND CENTER, SQUAWKED 7700, AND PROCEEDED TO KRBL, MY NEAREST AIRPORT IN SIGHT, AND EXECUTED A POWER OFF SPIRAL OVER THE AIRPORT, EVEN THOUGH THE ENGINE WAS STILL CAPABLE OF PRODUCING SOME POWER. I LANDED NORMALLY 10 MINUTES AFTER THE INCIDENT STARTED AND WAS ABLE TO TAXI UNDER MY OWN POWER TO THE RAMP, ALTHOUGH WITH A MUCH MORE PRONOUNCED VIBRATION ON THE GROUND. THE CAUSE OF THE ENGINE FAILURE WAS DETERMINED TO BE A STUCK EXHAUST VALVE IN THE FULL OPEN POSITION. AN ENGINE ANALYSIS WAS PERFORMED OF MANY PREVIOUS FLIGHTS AND NOTHING SUGGESTED A FAILURE WAS IMMINENT.