On May 12, 1994, about 1845 central daylight time, a Cessna 150F, N6872F, sustained substantial damage in a hard landing and nose- over following a loss of engine power on takeoff at a private airstrip near Chippewa Falls, Wisconsin. Neither the certified flight instructor, nor the student pilot were injured. No flight plan was filed for the instructional flight, and visual meteorological conditions prevailed at the time. The flight instructor and the student both stated the engine had a partial loss of power shortly after takeoff, and the airplane settled back to the runway. The nose landing gear sheared off and the airplane nosed-over. Two Federal Aviation Administration Inspectors examined and ran the engine. The inspectors found the carburetor heat control was actuated and could not be turned completely off. The carburetor heat door was catching on a nut which allowed the door to have a limited range of operation.