After completing a parachute drop, the pilot was returning to the airport for landing. The pilot applied carburetor heat while descending to the pattern, but turned it off during the extended down wind. When on short final, the engine suddenly lost power. The pilot was unable to land the airplane on the airport and initiated a forced landing in an open field. During the landing roll, the airplane nosed over. Examination of the engine did not reveal evidence of mechanical failure or malfunction.