On February 22, 1997, approximately 1300 mountain standard time, a Luscombe 8A, N77947, registered to and being flown by a private pilot, sustained substantial damage when the aircraft nosed over during landing roll following an emergency landing on Interstate 84 near Morgan, Utah. The pilot was uninjured. Visual meteorological conditions existed and no flight plan had been filed. The flight, which was personal, was to have been operated under 14CFR91, and originated from the Morgan County airport, Morgan, Utah, approximately 1245. The pilot reported that while approximately 1500 feet above ground level in cruise he attempted to lean the mixture by retarding the mixture control. During this process he ascertained that the mixture control would not move forward into a rich position. Rather, the mixture control would spring back out into an idle position. With the aircraft committed to a reduced power descent, the pilot executed an intentional emergency landing on the westbound lanes of Interstate 84 during which he had to maneuver over an over-pass and beneath power lines which crossed the highway. He reported that the landing was hard and during the rollout the aircraft departed the paved surface veering left across the median at which time the wheels struck a six inch lip at the edge of the pavement to the eastbound lanes. The aircraft then nosed over. There was no other property damage. Post-crash examination of the aircraft by an FAA inspector revealed that the wire-wound sheathing surrounding the mixture control push-pull cable had separated from the solid fitting end which is held firm at the back of the instrument panel. This disconnection allowed the mixture control cable and surrounding wire-wound sheath to flex but prevented the cable from changing the position of the mixture control at the carburetor.