On April 15, 2000, about 1415 Eastern Daylight Time, a Sidewinder, a homebuilt airplane, N13545, was substantially damaged while departing Cincinnati Municipal Airport, Cincinnati, Ohio. The certificated private pilot was not injured. Visual meteorological conditions prevailed and no flight plan was filed for the personal flight conducted under 14 CFR Part 91. The flight was destined for Capital City Airport, Frankfort, Kentucky. The pilot stated that he performed a normal run-up, and departed on Runway 21L. About 200 feet in the air, the engine ran rough and lost partial power. Between 30 and 50 feet above the ground, the airplane stalled and impacted terrain approximately 500 feet beyond the runway. The pilot added that he was not sure why the engine lost power, but suspected that it may have been "vapor lock" or a "clogged finger screen." According to a witness and Air Traffic Control Specialists, the accident flight was the pilot's second takeoff attempt. About 15 minutes before the accident, he aborted a takeoff due to "engine problems" and taxied back to the ramp. The pilot inspected brakes, fuel, oil, and the engine. After he could not find any anomalies, he taxied out for the second takeoff attempt. When first interviewed by a FAA inspector, the pilot failed to mention the first takeoff attempt. However, in a subsequent written statement, the pilot stated "the aircraft acceleration seemed slow" during the first takeoff attempt. When asked about the engine, the pilot stated that he did not know the date of manufacture, or total flight time of the engine. When asked why, the pilot stated that he purchased the airplane in 1984, and "there were no papers on the engine." He did know that it was overhauled sometime before 1984. An Air Safety investigator at Textron Lycoming was provided with the engine serial number. He stated that the serial number corresponded to a O-290-G1B engine that was manufactured during the 1940's and 1950's. The investigator added that the engine was not manufactured for use in aircraft. It was primarily sold to the military as a ground power unit for generators or pumps. According to FAA Flight Standards Service Release 462, the following components of the engine were considered ineligible for use in a certificated aircraft engine: connecting rod assembly, piston, piston pin, piston ring-top compression, piston ring-2nd compression, and oil regulating ring. However, the engine was not required to meet certification standards as it was used in an airplane with an experimental certificate. A FAA inspector examined the engine. He noted that the fuel tank was approximately 1/2 full, and a fuel sample did not contain any visible contamination. The fuel gascolator screen, carburetor inlet screen, and fuel bowl were absent of debris. The carburetor float needle and seat appeared to be in good condition. When the inspector rotated the propeller by hand, thumb compression was attained on all cylinders. Valve train continuity was confirmed, and all spark plugs produced a spark at the lead.