On September 27, 1998, approximately 1730 mountain daylight time, a Cessna 152, N4568F, was substantially damaged during a forced landing following a partial power failure near Durango, Colorado. The private pilot, the sole occupant in the airplane, was not injured. The airplane was owned/operated by Durango Air Service, Inc. under Title 14 CFR Part 91. Visual meteorological conditions prevailed for the local area personal flight which originated approximately 35 minutes before the accident. According to the pilot, he experienced a power failure and performed a forced landing to an open field. The field was wet and soft; the nose wheel landing gear failed aft during the landing roll, and the airplane nosed over. The vertical stabilizer and left wing were bent. Postimpact examination of the engine revealed that the castellated nut which held the throttle cable in place had backed off and separated from the airplane. The displacement of the throttle cable permitted the engine to transition to the idle position.