On September 19, 2003, about 1215 eastern daylight time, a homebuilt Capella Classic, N82536, registered to a private individual, experienced a partial loss of engine power and was substantially damaged during a forced landing near Port Charlotte, Florida. Visual meteorological conditions prevailed at the time and no flight plan was filed for the 14 CFR Part 91 personal flight. The private-rated pilot, the sole occupant, sustained minor injuries. The flight originated about 1130, from the Charlotte County Airport, Punta Gorda, Florida. The pilot stated that he preflighted the airplane which included checking the fuel tanks for contaminants; none were reported. He then started the engine and taxied the airplane to a fixed-base operator (FBO) were 5 gallons of 100 low lead fuel were added to each wing fuel tank which already contained about 3-4 gallons of automotive fuel. After fueling, he again checked the fuel tanks for contaminants; none were found. He started the engine, taxied to the runway, and performed an engine run-up before departure, no discrepancies were noted. He applied power, climbed to 20 feet above ground level, then landed on the remaining portion of the runway; there was no problem with the engine or airplane that caused him to land. He elected to depart from the same runway, and applied power from the spot where the airplane had been stopped from the previous landing. After takeoff, the flight remained in the traffic pattern where he performed two touch-and-go landings. After takeoff from the second, the flight proceeded away from the airport and approximately 20 minutes later when flying at 1,000 feet, the engine rpm dropped from 4,900 to 1,900. Attempts to restore engine power by cycling each fuel selector valve which had been in the "on" position, pumping the throttle, and checking the magnetos were unsuccessful in restoring engine power. He maneuvered the airplane for a forced landing which occurred in a vacant lot. After touchdown, the airplane cartwheeled, coming to rest upright in a shallow retention pond with the fuselage resting on the right wing, and a section of the left wing separated. Initial examination of the engine at the accident site by an FAA airworthiness inspector revealed no fuel at the mechanical fuel pump. Additionally, he did not smell fuel at the scene. The airplane was recovered for further examination. Examination of the engine by a representative of the engine manufacturer and an FAA airworthiness inspector approximately 1 week after the accident revealed the gascolator was found to contain fluid contaminated with water. Examination of the carburetor bowls revealed evidence of long term contamination by dirty fuel and water beading. The fuel slides of each carburetor was noted to be installed incorrectly related to indexing the slide to the throttle bore. Examination of the oil system revealed no evidence of oil starvation. Impact damage was noted to the propeller. At a later date, the engine was installed on a test stand and impact damaged components necessary for the attempted engine run were replaced. The engine was started and operated to 90 percent rated power with crushed exhaust pipes and no engine coolant. A noticeable vibration was noted during the engine run.