On May 29, 2010, approximately 0825 central daylight time, a Thunderbird Aviation Top Dog weight-shift control airplane, N2574V, impacted terrain during an attempted takeoff from the Carter Memorial Airport (T91), Luling, Texas. The sport pilot, the sole occupant, was seriously injured. The airplane's fuselage structure and wing surface were substantially damaged. The airplane was registered to and operated by a private individual under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed for the flight, which operated without a flight plan. The flight was originating at the time of the accident. There were no witnesses to the accident. The pilot could not recall the accident sequence and did not provide any statement. A review of the wreckage conducted by an inspector from the Federal Aviation Administration revealed no preimpact anomalies. The airplane was powered by a Rotax 503, twin cylinder, dual carbureted, two stroke engine, rated for 50 hp at 6,800 rpm. The engine was removed and sent to the engine manufacturer in Vernon, Canada, where an examination was conducted under the auspices of the Canadian Transportation Safety Board (TSB). All spark plugs were found to be finger-tight, signatures of water intrusion were detected in one carburetor float bowl, and the other carburetor bowl contained fuel tainted with an unknown substance. The engine was mounted on a test stand complete with clean fuel. The spark plugs were tightened and the air filter was replaced. The engine was turned over via a pull starter and started on the first pull. The engine operated normally at idle rpm, but when advanced towards maximum rpm, would only achieve 5,730 rpm. Repeated attempts confirmed that the engine could not reach maximum rpm. Further examination revealed that the engine spark plugs and carburetor main jets were not per the manufacture's specifications. After replacing them with components recommended by the manufacture, the engine was able to achieve 6,230 rpm, the maximum rpm expected for the conditions present during the test. According to the manufacturer, the carburetor main fuel jets that were installed at the time of the accident provided a lean mixture that would not allow the engine to reach its rated power.