On July 26, 2000, about 0930 hours Pacific daylight time, a Schweizer G-164B, N3629E, sustained substantial damage when it made a forced landing in a field and nosed over near Tracy, California. The forced landing was precipitated by a total loss of engine power. Haley Flying Service was operating the airplane under the provisions of 14 CFR Part 137. The commercial pilot was not injured. The aerial application flight departed a private strip near Tracy about 0900. Visual meteorological conditions prevailed and no flight plan had been filed. The pilot told the Federal Aviation Administration (FAA) accident coordinator that he lost engine power during cruise flight and landed in a tomato field that had just been irrigated. His touchdown was good but, as the wheels mired in the mushy dirt, the tail started slowly going up and continued until the airplane nosed over. A ground crewman stated the airplane fuel tanks were more than halfway full when the airplane departed. The FAA accident coordinator inspected the airplane. The fuel tanks were clean. The day after the accident, the engine ran without hesitation when it was started using the airframe systems. The carburetor was removed for inspection, and the teardown examination revealed no defects. No evidence of contamination was found in the carburetor bowl or the fuel lines. Both magnetos and the magneto switch functioned properly. Fuel was obtained from the operator's 10,000-gallon fuel tank for analysis. The sample analysis revealed the presence of a high boiling point hydrocarbon similar to lubricating oil. According to the laboratory report, this compound can result in gum deposits. The operator's fuel supply tank was drained for cleaning and no contaminants or gum deposits were found.