On June 5, 2006, about 1900 mountain daylight time, a Beech A36 Bonanza, N24BF, sustained substantial damage following a loss of engine power and subsequent forced landing approximately two miles south of the Ennis - Big Sky Airport (KEKS), Ennis, Montana. The airplane, which was owned by the pilot, was being operated as a visual flight rules (VFR) cross-country flight under the provisions of Title 14, CFR Part 91, when the accident occurred. The certificated private pilot, and three passengers, were not injured. Visual meteorological conditions prevailed for the cross-country flight that originated from Ennis just before the accident. The pilot's planned destination was Hamilton, Montana. The pilot reported that shortly after takeoff, the airplane lost engine power and he elected to land in an open field south of the airport. The airplane sustained substantial damage to the firewall during the landing. On June 27, 2006, representatives from the Federal Aviation Administration (Helena, Montana, Flight Standards District Office) and Teledyne Continental Motors examined the airplane at a hangar facility in Belgrade, Montana. Functional testing of the mixture control linkage revealed that the metering shaft rotated to the mechanical stop when actuated to the "idle cut-off" position; however, when the mixture control was actuated to the "full-rich" position, the metering shaft rotated approximately 40 percent of potential (full) travel and did not reach the mechanical stop. Further examination revealed the mixture control arm retaining nut was tight, however extensive wear to the serrated mating surfaces of the mixture control arm and metering shaft was observed. The serrated edges had worn smooth and fretting type damage was noted to both surfaces. Maintenance records showed that on March 9, 2006, the airplane's engine, a Continental IO-520, was removed and the airframe was retrofitted with a factory-rebuilt IO-550-B in accordance with Supplemental Type Certificate (STC) SA2200SW. The mixture control arm was shipped with the factory-rebuilt engine. The certified airframe and power plant (A&P) mechanic, who performed the maintenance on the airplane, stated the mixture control arm was installed in conjunction with the engine conversion. The airplane had accumulated approximately 35 hours since the conversion.