The helicopter began losing engine power during an aerial application run while maneuvering. The pilot attempted a landing in the field and the helicopter impacted terrain about 80 yards from an access road at the north edge of the field. The helicopter sustained substantial damage. The pilot was uninjured. Examination of the helicopter revealed that the carburetor heat control spring that keeps the carburetor closed was broken and its support rod had worn into the housing. The heat valve could only be opened about 1/3 and while corresponding cockpit control remained in the closed position. The fracture surface of the spring was consistent with fatigue. The spring had accumulated a total time in service of 832 hours. The pilot was a certificated aircraft mechanic who also performed maintenance on the helicopter. A National Transportation Safety Board Pilot/Operator Aircraft Accident/Incident Report was not received from the pilot.