On August 20, 1993, at 0815 central daylight time, a Piper PA-25-235, N4693Y, operated by Central Flying Service, Inc., Washburn, North Dakota, sustained substantial damage when it struck two fence posts during landing roll following a forced landing near Washburn. The commercial pilot was not injured. No flight plan was filed for the aerial application flight. Visual meteorological conditions prevailed at the time of the mishap. The pilot reported he was maneuvering to commence an application run when the engine lost all power. He made a landing in the field, but struck two fence posts during landing roll. The engine examination revealed the left magneto impulse coupling flyweight was jammed, and the magneto was seized. The impulse coupling was failed, and the magneto drive gear, and camshaft gears were damaged.