On December 22, 1994, about 2150 eastern standard time, a Piper PA-28-140, N41774, registered to Aviation Enterprises Unlimited, Inc., was substantially damaged during a forced landing near La Belle, Florida, while on a 14 CFR Part 91 personal flight. Visual meteorological conditions prevailed at the time and no flight plan was filed. The private-rated pilot and two passengers were not injured. The flight originated about 2055 from the Fort Lauderdale Executive Airport, Ft Lauderdale, Florida. The pilot stated that during cruise flight, the engine experienced a partial loss of power. Attempts to restore engine power were unsuccessful. Unable to maintain altitude he landed on a road and during the landing roll, the left wing of the airplane collided with a bridge. Examination of the engine by an FAA airworthiness inspector revealed that the No. 3 cylinder exhaust valve failed for undetermined reasons. The cylinder was overhauled on January 30, 1990, by the Harrison Engine Service, Inc., Work Order No. 8582. Scribed into the failed cylinder skirt was "11-89 HES 8582." Records from the overhaul facility were not available. Review of the airplane logbooks revealed that the airplane was last inspected in accordance with an annual inspection on December 9, 1994. Service Bulletin No. 388 dated May 13, 1992, which describes the procedures for checking the exhaust valve guide condition, requiring special tools, was not complied with. Compliance with the service bulletin is non-mandatory. The mechanic stated that he performed a general inspection for abnormal appearance and looseness. The engine had accumulated about 7 hours since the last annual inspection at the time of the accident.