PLT OF SMT MEL ACFT MADE A PRECAUTIONARY LNDG AFTER ENCOUNTERING ICING CONDITIONS AND A SUBSEQUENT ROUGH RUNNING ENG.

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DURING A FLT FROM MONROE; LA; TO LINCOLN; NE; IT BECAME NECESSARY TO DIVERT TO SPRINGFIELD; MO. IN CHKING THE WX BEFORE THE FLT; THE BRIEFER MENTIONED AREAS OF PRECIP ON LIVE RADAR AND AN AIRMET FOR POSSIBLE ICING OVER OUR RTE OF FLT. I ASKED FOR ANY PIREPS OVER THE RTE -- A FEW WERE GIVEN WITH SOME MENTION OF ICE BTWN 7000-9000 FT MSL. FIRST I DECIDED TO FILE A NORTHERLY COURSE UP TOWARDS ST LOUIS; THE NWESTERLY TO LINCOLN; NE; TO AVOID THE AREAS OF MODERATE RAIN WHICH HAD DEVELOPED OVER OKLAHOMA AND KANSAS. WITH WX RADAR ON BOARD; I FELT THAT NAVING THROUGH ANY AREAS OF LIGHT PRECIP WAS WITHIN MY CAPABILITY. AS FAR AS THE ICING; I DECIDED THAT FILING FOR 4000 FT WOULD LESSEN MY RISK OF GETTING INTO ANY ICING BY STAYING AT OR BELOW THE FREEZING LEVEL WHILE ENRTE. MY DECISIONS ON THE WX WERE ALLOWING FOR AN UNEVENTFUL FLT UNTIL I REACHED THE ARKANSAS-MISSOURI BORDER. THERE I STARTED TO PICK UP SOME LIGHT MIXED ICE WHILE FLYING IN SNOW SHOWERS. CHKING WITH ATC FOR ICING RPTS; STILL NONE WERE RPTING ICING OUTSIDE A 7000-9000 FT BAND. I TOLD THEM I WAS PICKING UP LIGHT ICE AT 4000 FT AND THEY INFORMED ME THAT THEY WOULD NEED ME TO CLB TO 5000 FT SOON FOR OBSTRUCTION CLRNC. I CONTEMPLATED REVERSING COURSE TO THE VFR CONDITIONS I KNEW EXISTED IN NEASTERN ARKANSAS; BUT INSTEAD DECIDED TO CLB TO A DIFFERENT ALT. WHILE CLBING THROUGH 10000 FT FOR 12000 FT; ONE OF THE ENGS BEGAN TO RUN ROUGH INTERMITTENTLY. AS SOON AS IT HAPPENED A SECOND TIME; I ASKED ATC FOR VECTORS TO SPRINGFIELD; AND ADVISED THEM OF THE ENG ROUGHNESS. I DECLINED DECLARING AN EMER; AS BOTH ENGS WERE RUNNING SMOOTH NOW THAT I HAD REDUCED PWR FOR THE DSCNT. I SET UP AND EXECUTED THE ILS FOR RWY 2 AND LANDED WITHOUT INCIDENT AT SPRINGFIELD. IN LOOKING BACK AT THIS; SEVERAL THINGS COME TO MIND THAT AFFECTED THE FLT AND LED TO AN ABNORMAL SIT. FIRST; I SHOULD HAVE MADE A NO-GO DECISION BASED ON THE AIRMET FOR ICING AT SUCH A BROAD RANGE OF ALTS. WITHOUT AN ACFT FULLY EQUIPPED AND APPROVED FOR KNOWN ICING; I SHOULD HAVE ALLOWED FOR A WIDER MARGIN OF SAFETY BY FLYING A DIFFERENT RTE OR WAITING UNTIL THE SYS HAD PASSED. SECOND; I SHOULD HAVE TRUSTED MY FIRST INSTINCT TO TURN AROUND AND FLY BACK TO VFR CONDITIONS AND LAND. ANOTHER OPTION WOULD HAVE BEEN TO LAND AT SPRINGFIELD WITHOUT ATTEMPTING TO CLB TO AN ICE-FREE ALT ONLY TO BE FORCED TO LAND AT SPRINGFIELD LATER. LASTLY; I LEARNED A GOOD LESSON ON NOT MAKING A WX DECISION WHEN I DON'T HAVE ALL THE INFO. I CHK WITH FLT WATCH SEVERAL TIMES DURING THE FLT FOR CURRENT DATA. HOWEVER; JUST BECAUSE NO ONE IS RPTING ICE DOESN'T MEAN IT'S NOT THERE. ALL THE CONDITIONS WERE RIGHT FOR ICE. I SHOULD HAVE NOTICED THAT.