On February 23, 2017, about 1730 Pacific standard time, an amateur-built Guarino, Vans RV-8 airplane, N470LM, was substantially damaged during a forced landing in a field in Marysville, California. The airline transport pilot and passenger sustained minor injuries. The airplane was registered to and operated by the pilot as a Title 14 Code of Federal Regulations Part 91 personal flight. Visual meteorological conditions prevailed, and no flight plan had been filed for the cross country flight. The flight originated from Half Moon Bay Airport, Half Moon Bay, California, about 1600 with a planned destination of Auburn, California. The pilot stated that he departed with full fuel in the wing tanks earlier that day from an airport in Davis, California. He landed in Half Moon Bay for lunch and departed about 1600 with a planned destination of Auburn. As he approached the vicinity of Auburn, he decided to divert over to the Oroville Dam area to sightsee. He maneuvered around the area at about 2,000 ft above ground level (agl) and then started the leg back to Auburn. While en route, the engine experienced a loss of thrust and the pilot could feel the airspeed slow. The propeller was still windmilling, but the throttle had no effect on the engine. He noted that the propeller circuit breaker had popped out, but there were no other indications of an anomaly in the cockpit. The pilot further stated that with the engine operating at partial power, he was unable to maintain level flight and configured for the best glide speed. He attempted to restart the engine twice and switched fuel tanks, to no avail. The pilot made a forced landing in a soft field, resulting in the airframe sustaining substantial damage. The pilot estimated that at the time of the accident he had enough fuel on board for about 1.5 hours of flight time. A post accident examination and attempted engine operational test was completed. There was no apparent visible evidence of engine mechanical anomalies or thermal distress. Fuel continuity and pressure in the system was confirmed. There were several wires and a wiring harness disconnected in between the cockpit and the firewall; it is unknown if this happened after impact. A fuel source was rigged to the left fuel tank and investigators attempted to start the engine. The starter solenoid was heard to activate, but the engine would not start. An external source of power was used in attempt to start the engine and the engine cranked but would not start. A starting fluid was sprayed into the air intake, and investigators again attempted to run the engine; the engine temporarily started, but then quit after 1-3 seconds. The fragmentation of the electrical system precluded investigators from definitively determining the engine failure.