PLT OF AN SMA SEL DECLARED AN EMER AND DIVERTED TO ANOTHER ARPT DUE TO A ROUGH RUNNING ENG.

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ENG HAD SLIGHT MISS ON WAY FROM PWK TO J21. PLUGS CLEANED AT J21 -- RUN-UP OK. LEFT J21 AND AT ABOUT 30 MI NW OF COLUMBIA; ENG AGAIN HAD OCCASIONAL MISS. COULDN'T CLEAN MISS BY LEANING. ENG NOW CONTINUOUSLY MISSING. I ASKED APCH FOR VECTOR HEADING TO GET TO THE CLOSEST ARPT. TURNED TO HEADING AND TRIED MAGNETOS SEPARATELY AS WELL AS RPM CHANGES -- NO GOOD -- WITH ROUGH ENG IN UNFAMILIAR AREA RESPONDED 'YES' WHEN CTLR ASKED IF I WANTED TO DECLARE EMER. ACTUALLY TURNED OUT TO BE UNEVENTFUL. ENG CONTINUED TO RUN (BUT ROUGH) AND WE MADE ARPT WITH PLENTY OF ALT. AT 3000 FT CANCELED IFR AND MADE NORMAL LNDG. MECH CHKED ENG AND 1 CYLINDER HAD FOULED PLUGS (OK). CYLINDER PULLED AND SENT OUT FOR SVC. RINGS NOT BROKEN; ABOUT 600 HRS ON NEW CYLINDER. MECH SAYS IT LOOKS LIKE RINGS WEREN'T SEATING; RING TENSION GOOD AND NO SCORING IN CYLINDER WALL. ATC VERY RESPONSIVE AND HELPFUL IN GETTING US SAFELY DOWN. REPRESENTATIVE FROM APCH CALLED LATER (15 MINS) TO CHK IF EVERYTHING OK AND REPRESENTATIVE FROM FAA CALLED ABOUT 15 MINS LATER ALSO TO CHK IF EVERYTHING OK.