PLT ON A VFR NIGHT FLT IN AN SMA SINGLE ENG LAND PVT ACFT DECLARED AN EMER TO ATC AFTER LOSING CTL OF ACFT BY ITS INABILITY TO CLB AND 'BUFFETING' DUE TO PICKING UP 'RIME ICE' ON THE ACFT.

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WX RPT INDICATED 4500 BROKEN AT BUF. WX ENRTE -- CEILING UNLIMITED ENRTE; NO ICING RPTED. AT APPROX 50 NM E OF BUF AT 7500 FT AGL I COULD SEE THE STARS AND WAS PREPARING TO GO TO 9500 FT AGL WHEN I NOTICED SLIGHT RIME ICING ON MY OUTSIDE TEMP GAUGE. AT APPROX 9000 FT AGL; THE PLANE STARTED BUFFETING AND UNABLE TO CLB (MY PITOT HEAT WAS TURNED ON AT TKOF) AND AIRSPD FLUCTUATING. THE RIME ICING WAS INCREASING. CTLS BECAME DIFFICULT. I NOTIFIED ATC OF MY SIT AND REQUESTED 6000 FT. I DECLARED AN EMER; AND DSNDED TO 6000 FT. THE BUILDUP STOPPED BUT RIME WAS STILL PRESENT (TEMP INDICATED MINUS 3 DEGS C). I COULD SEE THE GND LIGHTS AND THE STARS AT THIS POINT AND REQUESTED AND GIVEN 4000 FT AGL. THE RIME ICE DISAPPEARED. I WAS THEN TRANSFERRED TO ROCHESTER ATC AND ASKED IF I WANTED TO CONTINUE FLT TO HPN. I AFFIRMED THIS AND CONTINUED. I CLBED BACK TO 7500 FT AGL. AFTER CHKING SYR AND BGM ATIS (RPTED CLR UNLIMITED AND 25 MI). AFTER LNDG HPN; I AGAIN CHKED THE WX AND THEY STILL RPTED (4500 BROKEN AT BUF) CLR ENRTE. EVIDENTLY; THIS WAS AN UNRPTED BANK OF WX. I KEPT THE WINGS LEVEL AT ALL TIMES AND WAS ABLE TO KEEP CTL THROUGH THIS SCARY SIT.