A C172 PLT HAD TO RETURN LAND WHEN HIS ENG STARTED ACTING UP. EMER DECLARED. ACFT EQUIP PROB SPARK PLUG INSULATORS BREAKING DOWN.

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OUR FLT HAD JUST COMPLETED A TOUCH-AND-GO ON RWY 23L AT IND. DURING THE CLBOUT WHILE ON A 140 DEG ASSIGNED HDG APCHING 2500 FT MSL; THE ENG VERY SUDDENLY BEGAN TO MISS BADLY; AS THOUGH ONE CYLINDER WAS NO LONGER FIRING. APPLICATION OF CARB HEAT HAD NO EFFECT. I DECLARED AN EMER AND INFORMED IND APCH OF MY INTENTION TO MAKE A L TURN BACK TO IND AND OF MY INTENTION TO LAND ON THE RECIPROCAL OF THE RWY ON WHICH WE HAD DEPARTED. THERE WAS SOME CONFUSION BTWN MYSELF AND THE CTLRS AT THIS POINT BECAUSE I COULD NOT THINK OF THE RWY NUMBER. I HAD THE CHOSEN RWY INSIGHT; HOWEVER; AND AS SOON AS THE PAINTED RWY NUMBER CAME INTO VIEW I ANNOUNCED MY INTENT TO LAND RWY 5R. WE WERE INSTRUCTED TO CONTACT THE TWR AND WERE EVENTUALLY CLRED TO LAND RWY 5R. WE WERE ABLE TO TAXI UNDER OUR OWN PWR. I HAD ENOUGH TIME DURING THE APCH TO ASSURE THAT NO TFC WAS ON FINAL FOR; OR ON DEP FROM RWY 5R/23L. JUST AFTER WE SHUT DOWN AT THE FBO; I DRAINED FUEL FROM EACH SUMP/DRAIN POINT. THE TINT INDICATED THE FUEL WAS OF THE PROPER GRADE; AND NO CONTAMINATION WAS PRESENT. NEXT I PERFORMED AN ENG RUNUP. MAGNETO DROP WAS 100 RPM ON EACH MAGNETO WITH NO ROUGHNESS. A CARB HEAT CHK SHOWED THE USUAL LOSS OF RPM AS HEAT WAS APPLIED AND THE RPM REMAINED STEADY AT THE REDUCED FIGURE FOR THE DURATION OF THE CARB HEAT APPLICATION. THE MECH LATER DETERMINED THAT BOTH SPARK PLUGS IN THE SAME CYLINDER HAD 'GONE BAD' BECAUSE THE INSULATORS WERE 'BREAKING DOWN' AT HIGH OPERATING TEMPS. ALL PLUGS WERE CHANGED/REPLACED AND THE ENG HAS BEEN RUNNING FINE EVER SINCE.