On August 10, 2000, about 1600 Pacific daylight time, a Hamer Lancair 235, N84LH, sustained substantial damage during an off airport forced landing following a loss of engine power near Chiloquin, Oregon. The airplane is owned by the pilot and was being operated as a personal flight under the provisions of Title 14 CFR Part 91, when the accident occurred. The private pilot, who was the sole occupant of the airplane, was not injured. Visual meteorological conditions prevailed and no flight plan was filed. There was no fire and no report of ELT activation. The flight originated from Chiloquin, Oregon. In a telephone interview and written statement, the pilot reported that the aircraft's engine "...began running rough" and he subsequently made a precautionary landing to a nearby airport. After landing, the pilot inspected the airplane and determined that the rough running engine was the result of improper ignition timing. A certified mechanic retimed the magnetos to fire 25 degrees before top-dead-center. After a ground run and recheck of the ignition timing, the pilot chose to test fly the airplane before continuing the cross-country flight. Shortly after takeoff, at an altitude of approximately 150 feet above ground level (AGL), the airplane again developed a rough running engine and the pilot initiated a forced landing to a nearby highway. The pilot stated that he planned on landing the airplane on a non-congested area of the highway; however, while maneuvering for the landing the airplane collided with a flatbed tractor-trailer combination that was traveling on Interstate Highway 97. The airplane came to rest on top of the forward most section of the trailer, against the tractor. The driver of the semi-truck was not injured. Post-accident inspection and teardown of the airplane, by the pilot and representatives for the Sacramento, California, Flight Standards District Office (FSDO), revealed that the engine's crankshaft was fractured and had separated between the number three and number four crankpins. The aircraft was powered by a modified Lycoming O-290-G series engine. According to the engine manufacture, the O-290-G series engines were designed and built specifically for ground power applications.