EMER DECLARED FOR ROUGH RUNNING ENG.

Narrative

IT WAS APPROX XA30 CST; THERE WASN'T ANY VISIBLE MOISTURE AND WE WERE CRUISING APPROX 20 MI E OF C32 AT 7500 FT MSL ENRTE TO E TROY. SUDDENLY; WITHOUT WARNING; THERE WAS AN INSTANTANEOUS RPM DROP FROM CRUISE (2450) TO 1500 RPM. ONCE IN A WHILE THE RPM WOULD JUMP TO ABOUT 2000-2100 RPM AND THEN FALL TO BELOW 1000 RPM. WE WERE ON A VFR FLT PLAN AND RECEIVING FLT FOLLOWING FROM MADISON APCH. I IMMEDIATELY MADE THEM AWARE OF OUR SIT AND REQUESTED RADAR VECTORS TO MADISON. I FIGURED THIS WAS PRETTY MUCH THE CLOSEST ARPT; IN ADDITION THEY HAVE EMER EQUIP AND RADAR TO ASSIST ME. I ALSO MADE THEM AWARE THAT; IF I LOST THE ENG COMPLETELY; I DIDN'T KNOW IF I COULD MAKE THE ARPT. MADISON INFORMED ME THAT THEY WOULD TRY TO RAISE THE LIGHTS AT C29 NEAR THE TOWN OF MIDDLETOWN WHICH IS 5-10 NM CLOSER THAN MADISON AND IN FRONT OF US. I TRIMMED THE AIRPLANE FOR BEST GLIDE SPD (ASSUMING THE WORST WOULD HAPPEN) WHICH IS 80 MPH AND STARTED LOOKING FOR A PLACE TO LAND. BEING NIGHT; ALL I COULD SEE WAS MADISON'S LIGHTS IN A DISTANCE AND COMPLETE BLACKNESS AROUND ME. I KNEW THERE WERE FIELDS BUT; WITHOUT BEING ABLE TO MAKE OUT THE TERRAIN FEATURES; I COULDN'T TELL WHERE THE PWR LINES; HOUSES; TREES; AND HILLS WERE. I STARTED THE EMER CHKLIST BY FIRST CHKING THE ENG GAUGES WHICH; EXCEPT FOR THE TACHOMETER WERE ALL READING NORMAL. I KNEW THAT WE HAD PLENTY OF FUEL BUT I DOUBLE CHKED ANYWAY. NEXT I PULLED THE CARBURETOR HEAT; AT WHICH TIME THE ENG BEGAN TO SPUTTER. I THOUGHT WE WOULD BE COASTING FROM THAT POINT ON. THINKING THAT SOME ENG PWR WAS BETTER THAN NONE; I PUSH THE CARBURETOR HEAT BACK IN. IN ADDITION TO THE ENG ALMOST COMPLETELY QUITTING SEVERAL TIMES WE HAD NOW DEVELOPED RADIO PROBS. TAKING INTO CONSIDERATION THAT I MIGHT COMPLETELY LOSE BOTH MY ENG AND RADIO; AND REMEMBERING ALL OF THE ACCIDENT RPTS THAT I HAD EVER READ; I DECIDED DECLARING AN EMER WAS THE BEST COURSE OF ACTION. I FELT THAT ANY AMOUNT OF PAPERWORK WAS WORTH POSSIBLY SAVING MY LIFE. I COMPLETED THE REST OF THE CHKLIST AND WAS ABLE TO STABILIZE THE RPM AT APPROX 1500-1700 RPM AFTER I SWITCHED THE FUEL TANKS FROM 'BOTH' TO 'L;' BUT NEVER BEING ABLE TO FULLY RESTORE PWR. WE LANDED SAFELY AT MADISON AND ATTEMPTED TO LOOK OVER THE AIRPLANE. THE NEXT DAY WE GAVE THE AIRPLANE A THOROUGH PREFLT. WE NOTICED THE OIL WAS DOWN 1 1/2 QTS; BUT THAT WAS ALL. WE REFILLED THE OIL; RAN THE AIRPLANE FOR APPROX 20 MINS ON THE GND AND RETURNED TO E TROY WITHOUT ANY FURTHER INCIDENT. NOTHING WAS FOUND WRONG WITH THE AIRPLANE; TO MY KNOWLEDGE. QUESTIONS: IF IT WAS CARBURETOR ICE; WHY DIDN'T THE ENG SLOWLY LOSE RPM INSTEAD OF THE SUDDEN AND COMPLETE LOSS? ONCE I PULLED THE CARBURETOR HEAT AND LET IT RUN; WHY DIDN'T THE ENG CLR? IF FUEL CONTAMINATION: NOTHING SHOWED UP IN STRAINER AFTER PREFLT CHK; AFTER I SWITCHED TANKS; THE ENG PWR ONLY INCREASED ABOUT 300-400 RPM FROM 1200 RPM TO APPROX 1600 RPM BUT THE PWR NEVER COMPLETELY RETURNED. WHY?