The pilot took off with a 50 gal load of water to calibrate the spray system. He stated that during a swath run, he heard a loud noise or whirring sound and initially thought there was a spray pump propeller failure. Immediately, he pulled up, started a left turn to land, and began dumping the load. At the same time, he realized that he had lost power. The pilot was unable to reach the runway. Just prior to touchdown in a cut-over area, the left wing struck a stump and the plane crashed. The engine separated from the aircraft and came to rest partially submerged in a drainage ditch. The carburetor had separated and the throttle control had failed in tension. The mixture control push-pull wire was not attached to the carburetor. An examination of the mixture control wire attach end revealed no signs of tensile loading or scratches on the wire; the attaching hardware could not be found.