On August 8, 1998, approximately 1800 central daylight time, a Crane Quicksilver Sport 2 registered ultralight, N4562G, was substantially damaged during a forced landing following a loss of engine power, near Glenn Heights, Texas. The non-certificated pilot, who was the only occupant, was not injured. Visual meteorological conditions prevailed, and no flight plan was filed for the Title 14 CFR Part 91 personal flight. The local flight originated from a private grass airstrip about 5 minutes prior to the accident. The aircraft "type" classification is an "ultralight", however, since it has a registration number and an airworthiness certificate, it does not meet the guidelines for an ultralight exemption under Title 14 CFR Part 103, and therefore, must comply with FAA regulations regarding homebuilt "airplanes." The pilot reported that, after takeoff, about 1 mile from the airstrip, the 65 horsepower Rotax engine began to run rough. He immediately turned back toward the airstrip. About 1/2 mile from the airstrip, the engine lost power, and the pilot maneuvered the aircraft into a field. During the landing, the vertical main spars (wing support tubes) were structurally damaged. The pilot stated that he believed that the engine problem was precipitated by his not replacing/adjusting the carburetor jets. He stated that the installed jets were more suitable for "higher" altitude operations. The aircraft, which had accumulated a total of 12 flight hours, had recently been transported from Colorado.