FORCED LNDG CAUSED BY CARB ICING.

Narrative

I TOOK OFF SOLO FROM I62 TO 79N ON SAT/XX/94 IN A CESSNA 150M. THE ACFT WAS TIED DOWN AT OPEN FIELD WITH TANKS HALF FULL. IN THE MORNING OF THE INCIDENT I DELAYED MY DEP BECAUSE OF HVY FROST ON ACFT. AFTER IT WAS MELTED; I REFUELED AND TOOK OFF AT XX30 EST. THERE WAS NO VISIBLE MOISTURE. I CLBED TO 4500 FT; LEVELED AND LEANED THE MIXTURE. THEN AROUND XY30 I CLBED TO 6500 FT AT JUST SHORT OF FULL PWR AND THE MIXTURE L AT THE PREVIOUS POS. LEVELED OFF AT 6500 FT WITH ENOUGH TERRAIN CLRNC. THEN I TRIED TO ADJUST THE MIXTURE. AS SOON AS I PUSHED THE MIXTURE FULL RICH TO SEE RPM CHANGE; ENG BECAME ROUGH. IMMEDIATELY THROTTLE WAS SET TO FULL PWR; BUT IT DID NOT HELP. THE RPM GRADUALLY WORSENED WITH FLUCTUATION. AT 1500 RPM I BEGAN DSCNT. THE THROTTLE WAS AT FULL PWR POS AND THE MIXTURE AT FULL RICH UNTIL TOUCHDOWN. AS THE RADIO WAS SET TO MAX GROSS WT CT; I CALLED IT BUT HAD NO RESPONSE. I THEN CALLED ON 121.5 THAT DUE TO ROUGH ENG I AM ATTEMPTING AN OFF-FIELD LNDG. IT WAS 4000 FT AGL SO THE XPONDER WAS LEFT AT 1200. AFTER 3 CALLS ON 121.5; AN AIRLINE ABOVE RESPONDED AND RECOMMENDED ZDC; WHICH HANDED ME TO ZOB. ZOB REQUESTED ME TO SQUAWK 7700 AND GAVE ME RADAR VECTOR TO 2G9. THE PROB WAS DISCUSSED WITH FBO AT 2G9 AND AT I62. WE CONCLUDED IT WAS NOT WATER IN THE TANK BUT CARB ICING. I DID A CAREFUL PREFLT CHK AND THEREAFTER HAD NO ENG ROUGHNESS UNTIL LNDG AT I62. I THINK IT'S BETTER TO HAVE CTR FREQS IN THE CT FREQ LIST IN THE SECTIONAL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: PLT WAS CALLED TO VERIFY IF HE HAD USED ANY CARB HEAT DURING THE TIME OF ENG ROUGHNESS. HE HAD NOT USED ANY HEAT AS THE SKIES HAD NO VISIBLE MOISTURE. IN HIS MIND HE COULD NOT HAVE CARB ICE. ALTHOUGH HE HAD WAITED FOR ICE TO MELT (OR FROST) FROM HIS ACFT BEFORE DEPARTING; IT MIGHT HAVE BEEN A CLUE FOR A MORE EXPERIENCED PLT.