On August 21, 1999, at 1015 central daylight time, a Piper PA-18-150, N90904, sustained substantial damage during a forced landing following a partial loss of engine power, near Granite City, Illinois. The pilot said the landing area was rough and uneven. The pilot and one passenger reported no injuries. The personal 14 CFR Part 91 flight was operating in visual meteorological conditions. No flight plan was on file. The local flight departed Creve Coeur, Missouri, time unknown. The pilot told officials that he did not know the nature or origin of the loss of engine power. Subsequent to the accident, but after the airplane was moved to a location away from the accident site, an inspection of the engine and airframe failed to reveal any mechanical anomalies. The top spark plugs were removed and had brown deposits on them. The wings had been removed and no fuel remained in the fuel tanks; however, what fuel was present in the lines appeared blue in color, and was free of contamination.