C182 PLT EXPERIENCES A ROUGH RUNNING ENG AND DIVERTS. AFTER A GAR WITH NORMAL ENG OP; PLT ELECTS TO CONTINUE ON TO THE ORIGINAL DEST. ENG MALFUNCTION WAS ATTRIBUTED TO CARB ICING.

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I HAD ASKED DEP FOR LOWER WHEN THEY COULD; EXPLAINING THAT MY WX BRIEF STATED THE FREEZING LEVEL IS AT 6000 FT. PHL APCH CALLED AN ACFT ABOUT 35 MI IN FRONT OF ME ASKING THEM IF THEY HAVE ANY ICING CONDITIONS AND THEY RESPONDED 'NEGATIVE ICING.' THEN ANOTHER ACFT IN THE AREA ADVISED THAT THEY WERE AT 7000 FT WITH NEGATIVE ICING CONDITIONS. WE CONTINUED OUR FLT AT 6000 FT AND AFTER BEING GIVEN TO HARRISBURG APCH; I AGAIN ASKED FOR LOWER. HARRISBURG APCH GAVE US 5000 FT AND WE CAME OUT OF THE RAIN SHOWERS W OF HARRISBURG; BUT STILL IN IMC. AFTER PASSING MUU; THE ENG STARTED TO RUN ROUGH. I IMMEDIATELY APPLIED CARB HEAT. MY NEXT SCAN SHOWED THAT THE RPM WAS PASSING 1600 RPMS. I DEACTIVATED THE CARB HEAT THINKING THAT I NEEDED THE RPMS TO MAINTAIN MY ALT. I THEN NOTIFIED CLEVELAND APCH AND ASKED FOR AN ILS APCH INTO ALTOONA WHICH WAS ABOUT 10 MI IN FRONT OF US. THEY ADVISED THAT ALTOONA WAS AT MINIMUMS; BUT A DEPARTING ACFT CALLED AND ADVISED THAT IT WAS REALLY 300 FT AND NOT 200 FT. THEY GAVE ME VECTORS TO THE ILS. I BECAME ESTABLISHED AND FOLLOWED THE GS AND LOC. MY TRAINING IN HIGH PERFORMANCE ACFT TOLD ME THAT 99.9% OF ENG MALFUNCTIONS HAPPEN WHEN THERE IS A CHANGE IN SETTINGS; SO I ELECTED TO STAY AT CRUISE SPD UNTIL I HAD THE ARPT ENVIRONMENT IN SIGHT. WE BROKE OUT AND I SAW THE ARPT AT MY 11 O'CLOCK POS AND DID AN S-TURN TO GET TO THE RWY AND ALSO AT THE TIME I STARTED BACKING OFF ON THE PWR AND APPLYING CARB HEAT WITH NO ROUGHNESS NOTED. UNFORTUNATELY; THIS TURNED OUT TO BE A DOWNWIND LNDG AND I DID NOT GET THE MAINS ON THE GND UNTIL PAST THE 2/3 MARK DOWN THE RWY AND IMMEDIATELY STATED I WOULD DO A GAR. I WAS ABLE TO MAKE MY XWIND AND DOWNWIND TURNS IN VMC. I FOLLOWED THE RWY LIGHTS; UNTIL I FOUND MYSELF IN IMC AGAIN. I THEN CALLED BACK TO ZOB AND ADVISED THEM OF THE MISSED AND ALSO THAT THE ENG WAS NOW RUNNING OK. THEY ASKED ME IF I WANTED TO DO ANOTHER ILS AT ALTOONA; AND I STATED THAT SINCE THE ENG WAS NOW RUNNING OK; THAT I WOULD LIKE TO CONTINUE TO MY DEST; BUT AS A PRECAUTION I ASKED WHAT THE WX WAS AT JOHNSTOWN. APCH ADVISED ME THAT JOHNSTOWN WAS WORSE THAN ALTOONA; BUT THE NEXT ARPT W WAS LATROBE AND IT WAS A LOT BETTER. GETTING CLOSER TO AGC; I WAS TURNED OVER TO PIT APCH AND WAS ASKED IF I WANTED TO LAND AT AGC. I RESPONDED THAT SINCE THE ENG WAS STILL RUNNING OK; I WOULD LIKE TO CONTINUE TO MY DEST. I WAS 'CLRED FOR THE APCH' AND FOLLOWED THE APCH COURSE TO RWY 8. AFTER I CAME OUT OF THE CLOUD DECK; I ADDED LOOKING FOR THE ARPT TO MY SCAN; AT WHICH TIME PIT APCH CALLED STATING I WAS 100 FT LOW. I REPLIED; 'CORRECTING' AT APPROX THE SAME TIME I SAW THE REIL'S. I ENTERED A L BASE TO RWY 8; THEN FINAL; BACKING OFF ON THE PWR A LITTLE AT A TIME. I MADE THE LNDG WITH NO PROBS WHATSOEVER; BUT ONCE WE WERE DOWN AND ROLLING OUT; I BACKED OFF THE LAST OF THE PWR AND THE ENG'S PROP STOPPED. THE SECOND TRY THE ENG STARTED WITH NO PROB AND WE TAXIED TO THE PARKING SPOT. THE NEXT DAY; 3 DIFFERENT A&P MECHS ALL AGREED THAT MY PROB WAS CARB ICING. WE RETURNED E THE NEXT DAY WITHOUT ANY FURTHER ITEMS OF INTEREST. LESSONS LEARNED: AFTER TALKING WITH A&P MECHS AND FLT INSTRUCTORS; I LEARNED THAT I NEED TO APPLY CARB HEAT EVERY 10-15 MINS DURING A FLT IN SIMILAR CONDITIONS AND ALSO TO INCREASE MY PERSONAL MINIMUMS FROM 500 FT TO 1000 FT FOR DEP AND ARR ARPTS.