On July 29, 2005, at 0700 central daylight time, an amateur-built Wilson Acrolite 1T, Canadian registry C-IAVT, piloted by a private pilot, was substantially damaged during a forced landing shortly after takeoff from ultralight runway 15 at Wittman Regional Airport, Oshkosh, Wisconsin. Visual meteorological conditions prevailed at the time of the accident. The flight was operating under the provisions of Title 14 Code of Federal Regulations (CFR) Part 91 without a flight plan. The pilot reported minor injuries. The flight was originating at the time of the accident and was destined for Sawyer County Airport (HYR), Hayward, Wisconsin. The pilot reported that he was departing the Experimental Aircraft Association (EAA) AirVenture 2005 convention when the accident occurred. The pilot stated that the engine "lost power" during initial climb after takeoff from ultralight runway 15. The pilot reported that the airplane contacted a tree during the subsequent forced landing and then impacted a front yard of a nearby residence. The pilot reported that normal engine operation is around 6,300 rpm during takeoff. The pilot stated that during initial climb the engine was only developing 5,900 rpm. The engine was examined by representatives of the National Transportation Safety Board and Federal Aviation Administration after the accident. Engine crankshaft and valve continuity was established by rotating the propeller flange. Both carburetors were disassembled. The magneto-side carburetor fuel bowl contained several small fragments of black material. The black material was consistent with degraded rubber from an automotive-style fuel line. The engine was outfitted with black rubber, automobile-style fuel lines. The power-takeoff carburetor fuel bowl was void of containments.