PLT OF A PIPER PA24 COMMANCHE ENTERED ICING AND TURB DURING ENRTE AND APCH CAUSING A ROUGH RUNNING ENG. IT WAS LEARNED DURING POSTFLT INSPECTION THAT IMPACT ICING OF THE ENG COWLING INLET TO THE ENG CARB CAUSED THE ROUGH RUNNING ENG.

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INBOUND TO RENO FROM TUS; FSS AND LATER FLT WATCH WAS CONTACTED FOR WX ADVISORIES IN RENO AND THE SURROUNDING AREAS; THE MOST RECENT BEING 20 NM BEFORE THE HAWTHORNE; NV; AREA. RENO WAS SHOWING 2000 FT AND 6 MI VISIBILITY; FALLON WAS 4000 FT AND 10 MI. DECISION WAS; IF RENO WAS NOT GOOD; THE FLT WOULD DIVERT TO FALLON; NV. IN THE HAWTHORNE AREA; ZOA COMMENTED THAT THE ACFT LEAVING RENO RPTED NO ICING ON CLBOUT AND NO OTHER PROBS FOR APCHS OR LNDGS TO RENO. AN IFR CLRNC WAS REQUESTED AND RECEIVED; CLB TO 13000 FT DIRECT FMG; APCH ILS RWY 16 RNO. STARTED THE CLB OUT OF 10500 FT. APCHING 12500 FT THE ENG BEGAN TO RUN ROUGH AND I ADVISED CTR OF THE DIFFICULTIES IN CLBING AND THE ENG WAS RUNNING ROUGH. CARB HEAT WAS APPLIED BUT THE ENG WAS STILL ROUGH. CTR AUTH A BLOCK ALT OF 12000-13000 FT. AT 13 DME FROM FMG; THE ENG COUGHED; CAUGHT A COUPLE SECONDS LATER BUT CONTINUED TO RUN ROUGH. ABOUT THIS TIME; THE FLT ENCOUNTERED MODERATE TO SEVERE TURB THAT CONTINUED INTO THE APCH PHASE. CTR DID THE HDOF TO APCH. THEY QUERIED ABOUT ICING; NONE OR VERY LITTLE WAS SEEN ON THE WINGS AND WINDSHIELD. THE ENG WAS STILL COUGHING AND WAS VERY ROUGH; ROUGH AIR WAS ALSO STILL A FACTOR. ILS WAS CAPTURED AND I BROKE OUT ABOUT 1600 FT; LNDG WITHOUT INCIDENT. POSTFLT INSPECTION BY MYSELF AND THE RAMP ATTENDANT SHOWED NO ICE ON THE WINGS; HOWEVER THE AIR INLET WAS COVERED WITH SLUSH AND ICE. THE SLUSH WAS ON THE FRONT FLAT PLATE. AROUND THE INLET AREA ICE/SNOW HAD PACKED IN ALMOST TO THE POINT OF PLUGGING IT COMPLETELY. A MODERATE BUILDUP WAS ALSO UNDER THE SPINNER ON THE COWL TIN BY THE STARTER AND IN BOTH CABIN AIR INTAKE SCOOPS. THE TURB ENCOUNTERED MUST HAVE DEVELOPED RAPIDLY OR WAS AT A LOWER ALT THAN THE ARRIVING JET TFC WAS USING. IN ANY EVENT THE WX ENCOUNTERED THAT EVENING WAS SEVERE FOR THOSE MINS THAT SEEMED VERY LONG.