ROUGH RUNNING ENG IN IMC. PLT ABLE TO MAKE SOME ADJUSTMENTS.

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ON IFR FLT PLAN FROM COLORADO TO ELDON; MO; WE HAD BEEN VFR ON TOP OVER MOST OF KANSAS CITY GOOD VISIBILITY AND TAILWINDS. NEAR SALINA; A MID-LEVEL LAYER DEVELOPED. WE ASKED FOR AND RECEIVED 15000 FT; WHICH PUT US IMC IN SNOW; BUT WITH COLD OUTSIDE AIR TEMP AND NO SIGN OF ICING. WE KNEW THE IMC AREA WOULD BE BRIEF (DEST CLR) AND VFR WAS AVAILABLE LOWER IF NECESSARY. AFTER 3/4 HR; WE BROKE OUT COMPLETELY; AND REQUESTED A GRADUAL DSCNT INTO DEST. IMMEDIATELY AFTER STARTING DOWN; ENG BEGAN RUNNING VERY ROUGH. I APPLIED CARB HEAT; BUT NO EFFECT. IT FELT AS IF MIXTURE WAS WAY OUT OF ADJUSTMENT. STILL WITH LOTS OF ALT; I ADVISED ATC (ZKC) OF THE SIT AS I SLOWED THE ACFT. EVENTUALLY I WAS ABLE TO ADJUST THE MIXTURE TO A MUCH LEANER POS AND RESTORE SMOOTH OP. IN THIS PROCESS; I WAS HANDED OFF TO WHITEMAN AFB APCH AND GIVEN LOWER ALT. AFTER STARTING DOWN AGAIN FROM 9000 FT AS REQUESTED BY ATC; THE ENG BEGAN BRIEFLY RUNNING ROUGH AGAIN. I TOLD ATC I PREFERRED TO STAY HIGH TO PRESERVE GLIDE OPTIONS IF SIT BECAME MORE SERIOUS. THEY ASKED ME TO CLB TO 9000 FT (OR DSND MUCH LOWER THAN I WANTED) TO AVOID MOA CONFLICT; AND WAS ABLE EVENTUALLY TO GET BACK TO 9000 FT. I OBSERVED A VERY NARROW; CRITICAL MIXTURE ADJUSTMENT BAND DURING THIS TIME. SINGLE PROBE EGT TEMP WAS NORMAL; BUT ONLY 1/2 TURN OF VERNIER RESULTED IN ROUGHNESS AT BOTH LEAN AND RICH SETTINGS -- UNUSUAL FOR MY ACFT. I CAME TO THE CONCLUSION THAT PERHAPS IMPACT SNOW OR SIMILAR HAD BLOCKED MY AIR INTAKE (ALTHOUGH I AM AT A LOSS TO EXPLAIN WHY ALTERNATE AIR VIA CARB HEAT SEEMED TO HAVE NO EFFECT). GND TEMP WAS 53 DEGS AT LNDG SHORTLY THEREAFTER (UNEVENTFUL) AND NO SIGN OF ANY ICE OR BLOCKAGE WAS OBSERVED ON GND. NO TRACE OF ANY IMPACT ICE OR SNOW; ETC; WAS EVER OBSERVED INFLT. SUBSEQUENT EXAMINATION AND FLT OF ACFT GAVE NO CLUE OF ANY PROB.