On July 19, 1997, at 1430 central daylight time, a Morgan Mini-500 helicopter, N500XM, was substantially damaged during a forced landing following a loss of engine power, near Ava, Missouri. Visual meteorological conditions prevailed at the time of the accident. The personal flight was operating under the provisions of 14 CFR Part 91 and was not on a flight plan. The pilot reported no injuries. The flight departed Cabool Memorial Airport, Cabool, Missouri at 1305 and was en route to Ava/Bill Martin Memorial Airport (AOV), Ava, Missouri, at the time of the accident. According to the pilot's written statement, the pilot was completing a series of flight tests that involved transitions from cruise flight into a hover. The pilot reported the exhaust gas temperature (EGT) was higher in cruise flight than when hovering and that the EGT cooled rapidly when the helicopter went from cruise flight into the hovering maneuver. The pilot stated that approximately two miles from AOV, at 2,500 feet mean sea level (msl), he began a 200 to 300 foot/min descent to reach the airport's traffic pattern altitude. The pilot reported that as soon as the descent was initiated he experienced unusual engine noises and an abrupt loss of engine power. The pilot stated that he lowered the collective and applied full power but received no indication of engine power. The pilot then initiated an autorotation to landing on a field with a 30-degree slope. The pilot stated that upon landing the aircraft rolled over and came to rest on its port side causing the rotor to impact the tail boom. Examination of the accident aircraft was conducted by the Investigator-in-Charge and representatives from Revolution Helicopter Corporation, INC., on August 18, 1997. Examination of the engine showed evidence that the engine had seized during flight. Inspection of the fuel metering system showed that the accident airplane had not complied with Revolution Helicopter Service Information Letter Number 042696, which requires the original factory carburetor jet setting to be adjusted to insure that the EGT be between 1,000 and 1,150 degrees Fahrenheit. No engine coolant was found in the coolant tank and the coolant hose tee was found to be deformed and leaking fluid.