THE PLT SAID WHILE IN NORMAL CRUISE CONFORMATION, THE ENGINE MISFIRED. HE APPLIED CARB HEAT AND THE ENGINE PERFORMED NORMALLY. HE REMOVED THE CARB HEAT AND IN ABOUT 2 TO 3 MINUTES THE ENGINE LOST SUBSTANTIAL POWER. CARB HEAT WAS THEN APPLIED WHICH RESULTED IN A GAIN IN POWER, BUT NOT TO NORMAL SPECIFICATIONS. THE PLT MADE A DECISION TO MAKE AN EMERGENCY LDG ON THE ONLY AVAILABLE SPACE. HE REDUCED THE POWER TO IDLE, APPLIED FULL FLAPS AND ATTEMPTED TO LAND ON A FIELD. HE FELT THE ENGINE HAD CLEARED ENOUGH FOR A POSSIBILITY FOR CONTINUATION OF THE FLIGHT BUT ELECTED TO CONTINUE WITH THE LANDING. THE ACFT LANDED ON A FIELD, RAN ONTO A ROAD AND STRUCK A TELEPHONE POLE WITH THE RIGHT WING. METEOROLOGICAL CONDITIONS WERE CONDUCIVE FOR THE FORMATION OF CARB ICE ACCORDING TO THE 'CARBURETOR ICING PROBABILITY CHARTS.' THE TEMP WAS 26 DEG F AND DEWPOINT WAS 3 DEGS F.