On June 21, 1999, approximately 0715 mountain daylight time, a Weatherly 620B, N2033N, collided with the terrain during an emergency landing following a partial loss of engine power near Tetonia, Idaho. The commercial pilot received minor injuries, and the aircraft, which was owned and operated by the pilot, sustained substantial damage. The CFR Part 91 position flight from an agricultural application landing site to the company airstrip was being conducted in visual meteorological conditions. The flight had been airborne for about one minute at the time of the occurrence. No flight plan had been filed, and there was no report of an ELT activation. According to the pilot, who failed to return the Form 6120.1/2, just after he leveled off about 200 feet above the ground, it seemed as if the engine lost all power without any previous sign of impending malfunction. Because there was no suitable open field in which to land, the pilot attempted an emergency landing in a nearby ravine. During the touchdown, the right wing came in contact with the rough/uneven terrain and spun the aircraft around. During this sequence of events, the main landing gear collapsed and the aircraft sustained substantial damage. During the investigation, the aircraft's fuel system, electrical system, air induction system, and ignition system were inspected. No anomalies or contaminants were found in those systems, and both the electrical and mechanical fuel pumps were found to function properly. The NAR9B carburetor was taken to Precision Airmotive Corporation for an operational check and teardown inspection, and no anomalies, internal damage, or contamination were found. In addition, the engine was subjected to a partial teardown inspection, during which it was discovered that the right magneto drive gear shaft had sheared through its most-forward oil supply orifice. Except for the sheared shaft, no other evidence of malfunction or anomalies that would have precluded normal engine operation were found.