CESSNA C172 PLT HAS A ROUGH RUNNING ENG; DECLARES AN EMER AND DIVERTS FOR LNDG.

Narrative

WE CROSSED OVER ZZZ AT 5000 FT; WITH ALL ENG INDICATIONS NORMAL; BUT AT ABOUT 5 MI E OF THE ARPT; THE ENG MADE A LOUD POP; AND BEGAN RUNNING ROUGH AND VIBRATING. I CHKED AND ENRICHED THE MIXTURE; CHKED FUEL STATUS AND THROTTLED BACK THE PWR TO LESSEN THE VIBRATIONS. HAVING PASSED OVER ZZZ; I KNEW THAT IT WAS THE CLOSEST ARPT FROM MY POS. I DECLARED AN EMER WITH ZZZ APCH. ZZZ APCH GAVE ME A TURN AND VECTORS TO THE ARPT FOR LNDG. ATC ASKED WHAT THE EMER WAS; SOULS ON BOARD AND FUEL STATUS. I RPTED THE ROUGH RUNNING ENG; AND THAT THERE WERE 2 SOULS AND 2 HRS OF FUEL REMAINING; WITH FUEL GAUGES INDICATING ABOUT A 1/2 TANK FOR BOTH L AND R SIDE TANKS. I HAD PLENTY OF ALT FOR THE GLIDE BACK TO ZZZ AND KEPT THE ENG RUNNING AT MINIMAL RPM TO MINIMIZE THE VIBRATION; YET PROVIDE PWR IF NEEDED. APCH CLRED ME FOR LNDG; AND THEN PASSED ME OFF TO THE TWR CTLR. THE TWR CTLR AGAIN CLRED ME TO LAND AND NOTIFIED ME THAT THE 'EMER EQUIP WAS ROLLING' (AS A FORMER FIREFIGHTER; I APPRECIATED KNOWING THAT EQUIP WOULD BE STANDING BY). AS I TURNED FOR FINAL APCH; I ADDED FLAPS TO HELP INCREASE THE DSCNT RATE; BUT WITH SURFACE WINDS AS STRONG AS THEY WERE; I HAD TO RAISE FLAPS AND ADD A TOUCH OF PWR TO MAINTAIN THE GS TO THE RWY. THE PLANE TOUCHED DOWN ON THE DISPLACED THRESHOLD PORTION OF THE RWY; AND JUST SHORT OF THE MAIN RWY THRESHOLD; WHICH WAS QUITE SUITABLE FOR A LIGHT ACFT. AS I ROLLED TO A STOP; THE ENG; WHICH WAS IDLING AT MINIMAL RPM; STOPPED. THE TWR CTLR ASKED ME TO TURN OFF AT THE NEXT TXWY; AND TAXI TO THE GA RAMP; BUT I WAS UNABLE TO COAST CLR OF THE RWY. I MADE AN ATTEMPT TO RESTART THE ENG; BUT SINCE THE ENG SEEMED TO BE STRAINING TO TURN OVER; I NOTIFIED THE TWR CTLR THAT I WOULD NEED A TOW TO THE RAMP. AFTER WE TOWED THE PLANE BACK TO THE FBO; WE NOTED THAT OIL WAS DRIPPING FROM THE ENG COWLING ONTO THE NOSEWHEEL. IT WAS NOT A STEADY FLOW; BUT A SLOW DRIP; JUST COVERING THE NOSEWHEEL FAIRING; BUT NOT ENOUGH TO MAKE A PUDDLE ON THE RAMP. AFTER PARKING AND SECURING THE PLANE; I PULLED OFF THE COWLING; AND TRACED THE SOURCE OF THE OIL BACK TO THE AIR-INTAKE BOX UNDER THE CARB. THE OIL WAS DRIPPING FROM A SMALL HOLE IN THE BOTTOM OF THE 'BOX;' MAKING ME BELIEVE THAT THE SOURCE OF THE OIL WAS FROM A CYLINDER WITH AN 'OPEN' OR OTHERWISE STUCK INTAKE VALVE. I ALSO CHKED THE FUEL IN THE TANKS; BY DIPPING EACH SIDE; WITH 12 (OF 19) GALS IN THE R TANK AND 11 (OF 19) GALS IN THE L TANK; WHICH INDICATED TO ME THAT FUEL QUANTITY WAS NOT AN ISSUE. I ARRANGED TO MEET WITH A MECH FROM THE FBO THE NEXT DAY. WE FOUND THAT THE REAR R CYLINDER (#3 ON THE LYCOMING 320-E2D ENG) WAS COLD. PULLING THE BOTTOM SPARK PLUG; IT WAS OILED FOULED. A THUMB COMPRESSION TEST SHOWED THAT THERE WAS NO COMPRESSION FOR THIS CYLINDER. PULLING THE TOP SPARK PLUG REVEALED THAT THE TOP PLUG WAS DAMAGED; AND THAT THERE WAS A PARTIAL RING OF METAL IN THE CYLINDER; JUST BEYOND THE SPARK PLUG BARREL.