A C172 PVT PLT LOSES ENG PWR AND LANDS HIS ACFT IN A FARMER'S FIELD WHEN DELAYING THE USE OF CARB HEAT DURING LOW ALT CRUISE IN LIGHT RAIN SW OF 1H2; IL.

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FLT STARTED IN APPLETON; WI. TRIP TO MEIGS FIELD WAS UNEVENTFUL. DEPARTED MEIGS AND FLEW VFR AROUND THE CHICAGO CLASS B AIRSPACE. RTE OF FLT WAS SW TO SALEM-LECKRONE; IL. WX WAS VFR WITH A CEILING OF ABOUT 3500 FT. AIR TEMP WAS ABOUT 80 DEGS F. FLT FOR PAST 1/2 HR HAD BEEN IN LIGHT RAIN WITH VIS OF ABOUT 7 MI IN HAZE AND RAIN. APPROX 20 MI SW OF 1H2 ARPT; THE ENG BEGAN TO RUN VERY ROUGH. RPM'S WENT FROM 2250 RPM TO 1500 RPM BACK AND FORTH A FEW TIMES. CHANGED TO R TANK -- NO CHANGE. CHANGED TO L TANK WITH SAME RESULT. BACK TO BOTH TANKS AND PULLED ON THE CARB HEAT. ENG RPM DROPPED TO 1200-1400 RPM AND WITH LITTLE HT AGL; A PRECAUTIONARY LNDG WAS STARTED. THERE WAS NOTHING TO THE FRONT BUT TREES. A 90 DEG TURN N WAS STARTED AND FIELDS WERE NOTICED TO THE R. A BASE AND FINAL LEG WERE SET UP FOR A LNDG IN A NEWLY CUT HAY FIELD. THE LNDG ROLL WAS ABOUT 400 FT IN SOFT GND. WE DROVE INTO 1H2 ARPT AND LOOKED FOR AN A&P TO LOOK OVER THE ACFT. NONE WAS FOUND SO WE TOOK ON AN ADDITIONAL 10 GALLONS OF NEW FUEL AND DRAINED THE WING TANKS AND INSTALLED THE NEW FUEL. WE DRAINED SOME OF THIS THROUGH THE SUMP AND AGAIN TESTED THE ENG. THE ENG RAN FINE FOR ABOUT 10 MINS DURING THE TEST. DUE TO WX CONDITIONS; CARB ICE LOOKS TO BE THE REASON FOR THE ROUGH RUNNING ENG. CARB HEAT WOULD HAVE WORKED TO CLR THE ENG; BUT DUE TO OUR LOW CEILING AND LACK OF LNDG SITES AHEAD; THE OFF-ARPT LNDG SEEMED THE SAFEST OPTION. THE FIELD WAS TOO DAMP TO ALLOW A TKOF ROLL SO WITH THE HELP OF LCL FARMERS; THE PLANE WAS MOVED TO THE BLACK TOP COUNTRY ROAD AND FLOWN OFF. AN ILLINOIS STATE PATROL PLANE SAW THE TKOF AND FOLLOWED THE PLANE BACK TO 1H2 TO FILE A FIELD RPT. THERE WERE NO TICKETS ISSUED AND NO WARNINGS GIVEN. THERE WERE NO TICKETS ISSUED AND NO WARNINGS GIVEN. I DO NOT THINK ANY FARS WERE BUSTED IN THIS LNDG. THINGS TO REMEMBER: 1) CONDITIONS WHICH FORM ICE. 2) WHEN FLYING LOW UNDER WX; ALWAYS KEEP FIELDS IN SIGHT. 3) DON'T FORGET TO FLY THE PLANE WHILE WAITING FOR THE ENG TO RECOVER. THE SAFEST PLAN WAS TO LAND AND FIX THE PROB ON THE GND.