On September 21, 1997, about 1050 eastern daylight time, a Cessna 150L, N6865G, was substantially damaged when it impacted tress during the initial takeoff climb from Garnsey Field, Easton, New York. The certificated commercial pilot was seriously injured. Visual meteorological conditions prevailed and no flight plan had been filed for the personal flight conducted under 14 CFR Part 91. In a written statement, the pilot said after performing a preflight inspection, he started the airplane and taxied to the south end of the runway to perform his run-up procedures. He further stated: "...The take-off RPM was around 2500, until I was about 500 feet past the end of the runway and climbing. The power dropped back to 2000 RPM with substantial vibration. I was unable to climb...I was unable to avoid the tops of trees in front of field for emergency landing." A friend of the pilot who witnessed the accident, stated: "...[I] waited for him to warm up the engine. It ran real rough at first....He warmed it up for 30 to 45 minutes...He said he was going to taxi to the other end of the runway and if it sounded OK he would takeoff. He took off North and did not seem to be gaining altitude and banked to the right and the next thing we heard was the crunch of hitting trees..." Examination of the airplane's engine by a Federal Aviation Administration Inspector revealed that the number three cylinder exhaust valve was stuck in the open position. A review of the airplane's log book by the FAA Inspector revealed the airplane had flown about 35 hours since it's most recent annual inspection, which was performed on July 13, 1994.