On August 5, 1995, about 1700 central daylight time, a Schweizer Aircraft Corporation G-164B, registered to and operated by Payne's Flying Service, Inc., collided with terrain during a forced landing near Quitman, Mississippi. Visual meteorological conditions prevailed at the time and no flight plan was filed for the 14 CFR Part 137 aerial application flight. The airplane was substantially damaged and the commercial-rated pilot, was not injured. The flight originated about 5 minutes earlier. The pilot stated that after reducing power to descend to initiate spraying, the engine lost power. He initiated dumping of the fertilizer and landed in an area cleared of trees. During the landing roll, the aircraft collided with a tree stump and nosed over. The engine was removed from the airplane for further examination which revealed no evidence of preimpact failure or malfunction. The propeller governor and fuel control units were removed from the engine and sent to the manufacturer's facility for bench testing, which revealed no evidence of preimpact failure of the propeller governor. Bench testing of the fuel control unit revealed that the fuel flow in pounds per hour (pph) was 41 and 45 pph below the minimum specification during the deceleration schedule test for two of the three test points. Additionally, fuel flow in pph was documented as being 26 pph less than specification at maximum power conditions. Review of records provided by the Woodward Governor Company revealed that the same fuel control was sent to them in August of 1994, by Intercontinental Jet, Inc., for bench testing. The records indicate that the testing was performed on August 26th, and the results indicate that the unit only met specification at 3 of the 16 test points. Specifically, the change in the two test points during the deceleration schedule test from 1994 to 1995 indicate that the fuel flow in pph decreased at test point 9.1 from 125 to 114 respectively. The minimum specified fuel flow in pph at that test point is listed as 155. The test point 9.2 during the deceleration schedule test for the 1994 and 1995 bench tests indicate a change from 72 to 58 pph respectively. The minimum fuel flow required at that test point is listed as 103 pph. Following the bench testing in 1994, per the instructions from Intercontinental Jet, Inc., personnel, the fuel control unit was returned to them as is with no tag. The unit was not overhauled.