On August 15, 1999, about 1300 Alaska daylight time, a tundra tire equipped Cessna 206A airplane, N4605F, sustained substantial damage during a forced landing in a remote area, about 85 miles east of Cordova, Alaska. The airplane was being operated as a visual flight rules (VFR) cross-country business flight under Title 14 CFR Part 91 when the accident occurred. The airplane was operated by Alaska Expedition Company, Cordova. The commercial certificated pilot, the sole occupant, was not injured. Visual meteorological conditions prevailed. The flight originated at the company's fishing lodge, about 2 minutes before the accident. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), on August 15, 1999, the pilot reported he was returning to Cordova from his fishing lodge. About 300 feet above the ground, the engine suddenly quit. The pilot switched fuel tanks, activated the engine boost pump, and began a turn into the wind. The engine did not restart. The pilot said he selected an open area of marsh for an emergency landing area. When the airplane touched down, the nose wheel separated from the nose wheel strut, and the airplane nosed down in the soft terrain. The airplane received damage to the engine, propeller, and the left wing. Following the accident, the airplane was ferried to Merrill Field, Anchorage, Alaska, by a mechanic/pilot from Ward's Aero, Anchorage. On September 8, 1999, the mechanic notified the NTSB IIC that he found corrosion, and blockage of the vent hole of the engine manifold valve housing. The manifold valve is comprised of a fuel inlet, a diaphragm chamber, and outlet ports for fuel lines to each cylinder. The spring loaded diaphragm operates a valve in the central bore of the lower chamber of the valve body. Fuel pressure below the diaphragm opens the valve to allow fuel to flow to each cylinder. A spring above the diaphragm retains the valve in the closed position, absent any fuel pressure. The upper chamber of the valve housing, containing the spring, is vented to the atmosphere. The mechanic/pilot said that when he landed in Anchorage at the completion of the ferry flight, he pulled the throttle to idle during the flair. At that time, the engine quit running.