PLT OF AN SMA SEL DSNDED BELOW ASSIGNED ALT DUE TO PARTIAL LOSS OF PWR. SUSPECT CARB ICE.

Narrative

WHILE FLYING S ON V143 ON AN IFR FLT FROM W54 IN MARYLAND TO BNA; NASHVILLE; TN; CRUISING AT 8000 FT IN A C172 WITH A 0360A4M ENG A SMALL LOSS OF PWR WAS NOTICED AND I WAS UNABLE TO HOLD ALT DROPPING 300 FT IN A MIN. ATC WAS INFORMED AND CARB HEAT WAS APPLIED. I WAS CLRED TO 6000 FT WHICH I DSNDED TO WITH THE CARB HEAT ON AND FULL PWR. THE ENG CLRED AND I ASCENDED BACK TO 8000 FT WITH NO FURTHER PROBS. THE ENG NEVER RAN ROUGH WITH JUST A SMALL LOSS OF PWR OF ABOUT 350 RPM NOTICED FROM AN INDICATED 2500 RPM TO ABOUT 2250 RPM. I HAVE NEVER EXPERIENCED THIS PROB IN THIS ACFT WHICH I HAVE OWNED FOR SEVERAL YRS. THE ENG HAS BEEN IN THE ACFT SINCE NEW FOR 500 HRS AND 4 YRS. I WAS NOT SURE WHAT WAS OCCURRING WITH THE ENG AND JUST WENT THROUGH A NORMAL ENG OUT CHKLIST; 'FUEL BOTH; MIXTURE FULL; CARB HEAT ON; MAGS BOTH; PRIMER IN' AND PROB CLRED. WELL RESTED AT TIME OF INCIDENT. L ENG AT FULL PWR BECAUSE A SPEAKER ADVISED AT A SEMINAR SEVERAL MONTHS AGO THAT CARB ICE CLRS QUICKER AT FULL THROTTLE. TEMP 31 DEGS F CRUISING JUST 500 FT ABOVE A BROKEN CLOUD LAYER.