On July 22, 1995, at 1202 mountain daylight time, a Piper J3C-65, N5985V, sustained substantial damage during a forced landing approximately 1 mile from Centennial Airport, Englewood, Colorado. The pilot was not injured. Visual meteorological conditions prevailed for this local area personal flight and no flight plan was filed. According to the pilot, when he reduced power following initial climb, the engine decelerated to about 1,500 rpm and began to produce black exhaust smoke. He could not achieve any more power so he said he shut down the engine and selected what appeared to be a suitable off airport landing site. During landing roll, the left main landing gear collapsed. The left lower longeron, propeller, belly skins, and landing gear sustained damage. The pilot said the terrain was rougher than it appeared. The engine was examined by Jensen Aircraft, Salida, Colorado. According to the person who conducted the examination, he found the number four exhaust valve stuck in the open position. His statement is attached.