On July 7, 1999, at 1400 central daylight time, a Cessna 150M airplane, N7660U, was substantially damaged during a forced landing following a partial loss of engine power during takeoff from the Nick Wilson Field Airport, near Pocahontas, Arkansas. The flight instructor and the student pilot receiving instruction were not injured. The airplane was registered to and operated by the flight instructor, who was doing business as Swink Aviation of Pocahontas, Arkansas. Visual meteorological conditions prevailed for the Title 14 CFR Part 91 local flight for which a flight plan was not filed. The instructional flight originated from the same location approximately 20 minutes prior to the accident. The 3,765-hour flight instructor reported that the student pilot receiving instruction had accumulated a total of 5 flight hours. The flight instructor elected to stay in closed traffic at the Nick Wilson Field Airport (M70), to practice takeoffs and landings. During the fifth takeoff of the afternoon, a partial loss of engine power occurred during initial takeoff climb from runway 36. The flight instructor stated that, suspecting carburetor ice, he pulled the carburetor heat lever to the full hot (out) position, and the knob, with a portion of the control cable still attached, came all the way out. The flight instructor stated that he assumed the controls. He added that the power available was not sufficient to maintain a climb rate. The flight instructor maneuvered the airplane to avoid trees and obstacles until he was able to execute a forced landing to an open field. During the landing roll in the soft dirt, the airplane nosed over and came to rest in the inverted position approximately 1/4 mile from the departure end of runway 36. Examination of the airplane by the flight instructor revealed that the fuselage, aft of the baggage compartment, sustained structural damage. Examination of the carburetor heat control cable confirmed that the control cable was fractured, and the carburetor heat lever was found in the full "on" position. Based on the physical appearance of the control cable, the flight instructor suspected that the cable was "original equipment." A review of the maintenance records by the FAA inspector revealed that the airplane had accumulated a total of 6,837 flight hours since new.