While cruising at 3000 feet, the aircraft suddenly encountered strong turbulence. Almost immediately afterwards, the engine commenced to run roughly and the pilot was unable to maintain height. He intended to carry out a precautionary landing on a freeway, but then sighted a strip nearby and positioned the aircraft for a left circuit. He subsequently advised that he elected not to lower the landing gear because he considered that the aircraft would have rolled beyond the end of the 550 metres long sealed strip. The aircraft touched down some 350 metres beyond the threshold and slid for 134 metres before coming to rest. Investigation disclosed that the exhaust valve guide clearances in five cylinders were less than the minimum specified by the engine manufacturer. The engine power loss resulted when an exhaust valve in one cylinder became jammed in the valve guide by a build-up of carbon deposits. The loss of power occurred over unfavourable terrain, and the pilot had been committed to a downwind approach to the strip selected for the forced landing.