On February 8, 1997, at 1515 central standard time, a Cessna 172, N13611, operated by Shamrock Aviation, Ltd., sustained substantial damage when it impacted trees during a forced landing in West Chicago, Illinois. The private pilot and one passenger reported no injuries. The local, personal flight originated about 1513 and was conducted under 14 CFR Part 91. Visual meteorological conditions prevailed and no flight plan was filed. During a telephone interview, conducted on March 18, 1997, the pilot reported that he performed a "normal" run up and "nothing was abnormal" during the takeoff on runway 33. At an altitude of 500 feet above ground level, the engine "began to run roughly and lose power." He reported that he had to lower the nose to maintain an airspeed of 75 knots. He "turned back toward the field, established best glide, and called the tower." He checked the engine controls. He reported that he felt heat on his legs and smelled "something hot". No smoke or flames were visible. He attempted to land on runway 19L but "excess altitude, airspeed, and a downwind approach made it impossible... ." He advanced the throttle to full which "resulted in almost no acceleration." The pilot reported that he elected to land in a farm field south of the airport. He conducted a soft field landing. The airplane impacted a row of trees and bushes "at the end of the field." Examination of the airplane revealed engine controls were intact. Fuel samples were clear and clean. The engine operated normally during a warm up, run up, and a full power test run.