On July 13, 1996, at 0930 mountain daylight time, an Sauter Avid Flyer HH, N399S, sustained substantial damage during an emergency landing following a loss of engine power. The accident occurred 14 miles north-northwest of Stoneville, South Dakota. The pilot reported no injuries. The passenger suffered minor injuries. The personal 14 CFR Part 91 flight was operating in visual meteorological conditions. No flight plan was on file. The flight departed five miles north of Rapid City, South Dakota, at 0730 with the intended destination of Faith, South Dakota. The pilot stated that the engine lost power with "no warning." He was unable to determine the reason for the loss of power. He said that the landing was hard and on a "rugged" field. An examination of the airplane and engine by an inspector from the Federal Aviation Administration failed to reveal the nature of the power loss. The inspector commented that the owner/pilot had adjusted the valves on the day prior to the power loss and accident. The inspector said that there was fuel present after the accident. He said that all damage he noted was from impact and he could find no reason for the engine not to operate.