On July 28, 2013, at 1150 central daylight time, an amateur built experimental Phelps KitFox III, N873RX, collided with the terrain during an off airport forced landing following a loss of engine power in Oshkosh, Wisconsin. The pilot and passenger were not injured. The airplane received substantial damage to the airframe. The airplane was registered to and operated by a private individual as a 14 Code of Federal Regulations Part 91 personal flight. Visual meteorological conditions prevailed for the flight, which was not operating on a flight plan. The last leg of the cross-country flight originated from the La Porte Municipal Airport (PPO), La Porte, Indiana, at 0930. The flight was en route to EAA AirVenture at the Wittman Regional Airport (OSH), Oshkosh, Wisconsin, when the accident occurred. The pilot stated they were on the Fisk arrival, about 6 miles west-southwest of OSH at 1,800 feet above mean sea level, when the engine began running rough. The pilot was able to regain engine power twice prior to the engine losing all power. The pilot performed a forced landing in a nearby plowed field. The nose of the airplane contacted the terrain during the landing, pushing the engine rearward and buckling the structure under the forward cockpit area. The pilot had fueled the airplane, filling both tanks, prior to departing PPO. A postaccident examination of the airplane revealed the left wing fuel tank was nearly full of fuel while the right wing fuel tank and the header tank were both empty. When the fuel line connecting the left wing fuel tank to the header tank was moved, fuel began draining from the wing into the header tank. The fuel line was removed and it was noted that the line had softened at the installation bend leading from the wing tank allowing the line to kink shutting off the fuel supply from the left wing tank. The pilot installed the fuel lines from both wing tanks to the header tank about 13 flight hours prior to the accident. The line from the right tank was intact and functioned normally. The lines installed were Gates automotive 50 pounds per square inch (psi) WP 5/16-inch diameter lines. A representative from KitFox stated they have not seen this problem with the fuel lines in the past. The fuel system on newer KitFox models has been modified to include a 90 degree elbow fitting which alleviates the tight bend in the fuel line from the wing tank to the header tank.