On September 18, 1994, at 1130 central daylight time, a Harris Dragonfly homebuilt, N345DH, was destroyed during a forced landing near Woodville, Texas. The private pilot/owner/operator sustained serious injuries. The local personal flight was conducted under visual meteorological conditions. The following information was obtained from the pilot/operator report. The pilot was on a maintenance flight to check the engine, carburetor, and a new header tank fuel sight gauge. The fuel float in the sight gage indicated the header tank was full; the pilot then shut off the fuel transfer pump to watch for float movement. The engine stopped; the transfer pump was turned back on, but the engine would not restart. Altitude was insufficient to reach clear ground so a landing was made in a wooded area. The airplane came to rest approximately 60 feet above the ground in a tree. Examination of the fuel system by the pilot revealed that the fuel float in the header tank fuel sight gage was jammed in the full position.