On August 3, 1997, approximately 1030 mountain daylight time, a Piper J3C-65, N5985V, was substantially damaged when it collided with terrain during a forced landing 4 miles south of Castle Rock, Colorado. The private pilot, the only occupant aboard, was not injured. Visual meteorological conditions prevailed, and no flight plan had been filed for the personal flight that was conducted under Title 14 CFR Part 91. The flight originated at Englewood, Colorado, approximately 1000. According to the pilot, he was in cruise flight at 7,500 feet msl (above mean sea level) when the engine rpm (revolutions per minute) dropped from 2,350 to 1,700. He said that at that altitude, a minimum of 2,150 rpm is required to maintain flight. He configured the airplane for best glide and landed in an open but uneven field. The main landing gear was sheared off and the fuselage buckled. Postaccident examination disclosed the no. 4 cylinder exhaust valve was stuck open. According to the Teledyne Continental Motors representative who examined the engine, it took "a large hammer to drive the valve out." The valve stem and guide were coated with "gummy black deposits." The pilot, who is an aeronautical engineer and holds an A&P (airframe and powerplant) mechanic's license, said the same situation occurred on July 22, 1995 (see FTW 95-L-A312). Engine examination following that accident revealed the no. 4 cylinder exhaust valve was similarly stuck open, and was attributed to excessive carbon deposits from high octane aviation gasoline usage in a low compression engine.