On May 26, 2017, at 1720 eastern daylight time, an Amstutz BD-5B amateur built airplane, N604KA, was involved in an off airport forced landing, following a loss of engine power in Portland, Indiana. The pilot was not injured. The airplane was substantially damaged. The airplane was registered to and operated by an individual under the provisions of 14 Code of Federal Regulations Part 91 as an engineering test flight. Visual meteorological conditions prevailed for the flight, which was not operated on a flight plan. The local flight originated from the Portland Municipal Airport (PLD), Portland, Indiana, at 1650.The pilot reported the airplane had 5.8 flight hours since he completed building it and the purpose of the flight was to obtain rate of climb data. Following the 6th climb, while at an altitude of 3,500 ft above mean seal level, the engine began to run rough. The pilot entered a left downwind at PLD and descended for a landing approach. The pilot reported he switched fuel tanks and performed a magneto check, neither of which corrected the engine roughness. The pilot noticed the cylinder head temperature and the exhaust gas temperature for the No. 1 cylinder decreased, indicating the No. 1 cylinder was not firing. As he turned onto base leg, the engine lost all power. The pilot realized he was not going to be able to make it to the runway, so he selected a field in which to land. The field contained 3 ft tall vegetation and was soft from recent rain. About 170 ft into the landing roll, the landing gear collapsed, and the airplane ground looped which resulted in damage to the flaps, ailerons, and horizontal stabilator. The airplane was equipped with a 2-cycle, 2-cylinder Hirth 3203E engine. A postaccident inspection of the engine by the pilot revealed there were separated wires in the connectors between the engine control unit (ECU) and the fuel injectors for both the No. 1 and No. 2 cylinders. The wires for the No. 1 cylinder connector appeared to be crimped properly with the insulation reaching the crimped connector. The pilot tugged on the wire and about 1/2 inch of the copper wire came out of the insulation. The pilot removed the connector for the No. 2 cylinder and found a broken wire at the splice connector that he had installed the month prior to the accident. The pilot stated that the engine was about 15 years old, even though the engine had 15 hours of total operating time. It is unknown when the No. 1 cylinder connector was installed on the engine.