On May 27, 1993, at approximately 0745 central daylight time, a Grumman G-164A, N4593, was substantially damaged when it nosed over during a forced landing after experiencing a loss of engine power near Weiner, Arkansas. The airplane, flown by a commercial pilot, was on a local aerial application flight. There was no flight plan filed and visual meteorological conditions prevailed throughout the area. The pilot, the sole occupant, was not injured. According to the pilot, the engine experienced a partial power loss during the turn at the end of a spray run. He stated that he dumped the chemical load, but was unable to sustain flight with the power available. He subsequently executed a forced landing to an open plowed field. The field was rough and the airplane nosed over during the landing roll. Investigation revealed that the control linkage assembly bracket was loose which allowed the condition lever to make an uncommanded move and disturbed the propeller governor, pitch control governor, and fuel control unit settings. Of the three retaining screws, one was missing and the other two were found loose. The airplane's maintenance records indicated that the engine had been installed on April 1, 1993, at the same time the airplane received an annual inspection.