On May 11, 2023, at 1210 central daylight time, an experimental, amateur-built 5151 Mustang, N91026, was substantially damaged when it was involved in an accident near Hector, Minnesota. The commercial pilot was uninjured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 test flight. According to a Federal Aviation Administration (FAA) inspector, the pilot, who was not the builder of the airplane, had recently overhauled the engine carburetors and modified the airplane’s fuel system. The overhaul of the carburetors and the fuel system modification were not recorded in maintenance records. When the pilot purchased the airplane, it was equipped with an airframe ballistic parachute, which he removed, and installed a fuel tank in its place. The pilot calculated that the weight of the parachute was equal to the weight of the fuel tank and 2 gallons of fuel. Before the accident flight, the pilot tied the tailwheel of the airplane to an anchor point and ran the engine at a high power setting for about 28 minutes to test the modifications he had made to the fuel system. He also adjusted the ground-adjustable propeller to obtain 6,200 rpm. After the ground run, he decided to test fly the airplane. During takeoff, with the newly-installed fuel tank selected, the engine only attained 6,100 rpm. He stated that suspected something was wrong. There was another aircraft close to the airport, so he radioed the other aircraft notifying the pilot of engine issues and asked the other aircraft for its position from the airport. The other aircraft responded that it was about 3 miles from the airport. The accident pilot told the other aircraft to go ahead and land and that he would land behind him. As the accident pilot turned toward the runway, the engine lost total power and the pilot was unable to glide the airplane to the runway. The airplane touched down in a soft field about 250 feet from the end of the runway. The accident pilot recovered the airplane and placed it in a hangar without the knowledge of or permission from the National Transportation Safety Board (NTSB) or FAA. The airplane sustained substantial damage that included damage to the wing rib and lower spar cap at left main gear attach point. Postaccident examination of the engine by an FAA inspector revealed that the front carburetor was out of its mounting flange and the clamp securing it was loose enough that the carburetor could be placed back in the flange and removed without loosening the clamp. A portion of the left side engine cowl, near the carburetor, was torn outward and aft due to impact forces during landing. There were no other engine anomalies noted during the examination. The pilot stated that he thought the loss of engine power was due to fuel starvation and that there may have been a problem with the vent of the newly installed fuel tank. An NTSB Pilot/Operator Aircraft Accident/Incident Report, form 6120, was not received from the pilot. The previous owner and builder of the airplane did not make the required logbook entry for completing Phase 1 flight testing; therefore, operation of the airplane was limited to the flight test area listed in the operating limitations of the airworthiness certificate (within 25 nautical miles of Bateman Field, Chippewa Falls, Wisconsin, excluding class C airspace). The accident flight occurred outside of the flight test area.