The engine lost power during a ferry flight and a forced landing was made in a soft field, where the airplane nosed over. A post-crash investigation revealed that the carburetor fuel-inlet filter screen was mostly blocked with a lint-like material. A microscopic exam of the material revealed that it appeared to be cloth fibers. The source of the material was not determined. Further investigation revealed that the engine had experienced repeated, intermittent power losses over the previous week in spite of repeated efforts by company mechanics to locate the source of the problem. According to the mechanics, contamination was found in the fuel system during those efforts; the filters were cleaned and the fuel system was flushed. The intermittent power losses occurred again. Earlier on this flight, the engine began to surge and a precautionary landing was made. The mechanic accompanying the pilot reportedly found no problems and the flight was continued. The accident occurred about 1 hour later.