During arrival, the pilot entered a left downwind for runway 11 and proceeded to make a low pass over the strip to chase off cows. At approximately midfield, he initiated a climb and according to the passenger, when power was applied, the engine did not respond properly. A witness reported that he observed the aircraft begin a climbing right turn, and at that time, the engine sounded as if it were sputtering, then quit. This witness said the aircraft made a descending left turn and hit the ground; he believed the pilot was avoiding a wooded area. Another witness stated that the plane pitched down abruptly. The pilot received a head injury and did not remember details of the accident, but believed the carburetor heat was on and full flaps were extended. A check revealed the carburetor heat was off and the flaps were retracted. No precrash engine failures were found. The temperature and dew point were 68 and 61 degrees. The last annual inspection was on 12/6/79. No metal-to-metal latches on seat belts.