19980724046449A

(.4)After departing and climbing to a cruise altitude of 6,500 feet above mean sea level, the pilot made a power reduction to descend below a cloud layer. After reducing power the throttle became stuck. Unable to move the throttle, the pilot diverted to an alternate airport. With the engine still running the airplane touchdown on the runway, but the pilot was unable to stop the airplane. The airplane exited the departure end of the runway, traveled 75 feet, and came to rest on top of a hill. While landing, the pilot thought the throttle was in the idle position. After the accident he realized it was slightly open. Examination of the carburetor, which had 12 hours since overhaul, revealed that the throttle shaft could not be rotated by hand, and a punch and hammer was required to remove the shaft. The shaft was galled where it rested on the bushings. Both ends of the original shaft (part number cf 13-1521) measured .3105 of an inch in diameter. A new shaft supplied by the overhaul facility measured .3104 of an inch in diameter at both ends. Under the supervision of the NTSB investigator, the original throttle bushings were re reamed with the original router. Then, the new throttle shaft was inserted, but was difficult to rotate. After removing the old throttle shaft, the bushings were re reamed, using a new router the company had brought after the accident. Again, the old shaft was inserted, but this time it rotated freely. The butterfly valve was then installed, and the throttle shaft continued to rotate without difficulty. (-23) pilot reduced power from cruise setting in order to descend below clouds en route. When he attempted to reapply power, encountered stuck throttle. Unable to move throttle control either direction. Made emergency landing at Newport, NH while operating @ partial power. Over ran end of runway.