The plt stated the purpose of the flt was to drop parachute jumpers. The jumpers deplaned at 8500 ft msl and the plt reduced pwr for a descent and landing at the arpt of departure. He stated he did not apply carb heat during the entire descent and he entered the traffic pattern with reduced pwr.During final apch, he applied throttle but the eng would not respond, it would only run rough. The plt stated the acft was descending rapidly and it was obvious he would not be able to land on the arpt. He landed the acft in a plowed, muddy fld where it nosed over.