EMER DECLARED FOR PRECAUTIONARY LNDG AFTER DIVERSION TO ALTERNATE ARPT. CARB ICING SYMPTOMS IN CLR AIR.

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ON A PLEASURE FLT FROM NC14-SHILOH ARPT TO THE VIRGINIA MOUNTAINS. MY ENG MISFIRED ONCE. UPON ADDING CARB HEAT IMMEDIATELY THE ENG BEGAN TO RUN ROUGH VARYING IN RPM FROM 1700 TO 1000. I ROLLED FROM MY HDG OF 285 TO 060 TO HEAD FOR THE NEAREST ARPT (HILLSVILLE GALOX) 16.8 MI AWAY. AS MY SINK RATE WENT TO 500 FPM I REALIZED THAT THE ARPT WAS NOT WITHIN REACH AND TURNED TO 121.5 TO ADVISE OF MY LOCATION; ALT AND INTENTIONS. A ROAD HDG NE TO GALOX WITH FIELDS ON THE SIDE WAS MY INTENDED EMER LNDG AREA AND AS I WAS COMMUNICATING THIS TO 121.5; MY ENG BEGAN TO VIBRATE AND THE RPM WENT TO 300-400. AS A FORCED LNDG WAS IMMINENT; I ROLLED THE AIRPLANE PARALLEL TO THE ROAD TO LINE UP ON THE FIELDS. AT ABOUT 1000 AGL THE ENG CAME BACK TO 1700 RPM SO I FOLLOWED THE ROAD BACK TO GALOX. THE ENG (RUNNING ROUGH FROM FULL PWR AND CARB HEAT) BECAME STRONGER GETTING TO 2100 RPM WITH A CLB RATE OF 200 FPM. THE ROAD RAN THROUGH GALOX AND HEADED NE BACK TOWARD I77 SO I FOLLOWED THE ROAD UNTIL I HAD THE ARPT WITHIN SIGHT. I NOTIFIED ATC ON 121.5 THAT I WOULD LAND ON RWY 18 OVERFLEW THE ARPT AT APPROX 5000 +/- AND CALLED CTAF 122.8 TO ADVISE LCL TFC OF MY INTENTIONS. TURNING ON FINAL ABOUT 2 MI OUT AND 2000 HIGH I BEGAN TO SPIRAL DOWN STAYING ON FINAL NOT ADJUSTING ANY ENG OR FLAPS UNTIL THE RWY WAS MAKEABLE. THEN I PULLED THE PWR BACK DROPPED FLAPS SLOWED THE AIRPLANE DOWN AND LANDED. TAXIED TO THE RAMP I THEN CUT THE CARB HEAT OFF AND THE ENG RAN JUST FINE. WHEN I GOT TO THE RAMP A PHONE CALL FROM ATC MR X CALLED TO CHK ON MY CONDITION AND ASKED FOR RPT FOR WHAT HAD HAPPENED INFLT. AFTER THE MECH AND TWO OTHER PLTS HAD CHKED THE FUEL WE DID 3 ENG RUNUPS; AND FOUND NOTHING WRONG WITH THE ACFT. WE DETERMINED THAT I HAD INCURRED ICE UNDER CLR AIR CONDITIONS. THE OUTSIDE AIR TEMP WAS 62 DEGS AND AT ALT WAS APPROX 40 DEGS; VISIBILITY 25 MI; NO CLOUDS. THE ICE OCCURRED OVER THE RIVER JUST W OF GALOX; VA. USING THE EMER PROC; TALKING TO SOMEONE ON 121.5 TO ADVISE OF MY SITUATION AND 'JUST KEEP FLYING THE AIRPLANE' SAVED THE DAY. CLR AIR ICING IS THE ONLY ANSWER THAT COULD BE ADDRESSED FOR THE SITUATION THAT OCCURRED AND THE RESULTING ENG RETURNING TO SVC WITH NO PROBLEMS JUST MINS LATER.