On July 11, 1996, approximately 1015 mountain daylight time, N3086G, a Hanks Avid Flyer amateur-built experimental airplane, sustained substantial damage during a forced landing northwest of Spanish Forks, Utah, airport. The private pilot and his passenger were uninjured. The local flight, operated under 14 CFR 91, was conducted in visual meteorological conditions. The ELT functioned, but was turned off by the pilot after the accident. The pilot stated that he had departed the airport with about 8-10 gallons of gasoline aboard. The aircraft had not climbed at his anticipated rate. He said that about three miles from the airport, he made a turn to attempt to return to the field. He said that the engine changed pitch at that time and the rpm dropped off, from about 6000 rpm to 4000 rpm. He said the aircraft could not sustain level flight. He continued a descent to a landing. The airplane nosed over during the landing roll, sustaining substantial damage. After the accident, the pilot performed a compression check on the engine, and found it to be normal. He found no evidence of a mechanical malfunction, nor was any evidence of a malfunction found by FAA inspectors. The empty weight of the airplane was 432 pounds, with a maximum gross weight of 1125 pounds. The pilot weighed about 284 pounds; he estimated the passenger's weight as 160-165 pounds; gross weight at the time of the occurrence was approximately 940 pounds. Density altitude was about 6700 feet.