On May 19, 1999, approximately 1145 Pacific daylight time, a Grumman G-164B, N6861K, nosed over in a freshly-plowed field during a forced landing near Okanogan, Washington. The pilot was not injured, but the aircraft, which was owned and operated by Okanogan Air Service, sustained substantial damage. The local 14 CFR Part 137 aerial application flight, which had been airborne for about 30 minutes, was operating in visual meteorological conditions. No flight plan had been filed, and there was no report of an ELT activation. According to the pilot, while maneuvering over an orchard he was about to spray, the aircraft encountered a "big bump of air" which pulled him up against his seatbelt and shoulder harness. About two seconds later, the engine suddenly lost power, momentarily sputtered, and then quit altogether. The pilot elected to make a forced landing in an Alfalfa field that was straight ahead, so he dumped his remaining chemical and prepared to land. The aircraft initially touched down in the alfalfa, but bounced back into the air and touched down again in a freshly plowed portion of the field. After this second touchdown, the main gear sank into the soft dirt and the aircraft nosed over. During a post-accident engine run, the engine would not start until the carburetor was tapped several times with a hammer. After jarring the carburetor with the hammer, the engine was able to be started and run. Disassembly of the carburetor did not reveal whether the float had stuck, contamination had temporarily blocked an orifice, or whether there was some other malfunction.