On December 7, 2020, at 1528 central standard time, a Vans RV-4 airplane, N423WH, was substantially damaged when it was involved in an accident near Sadler, Texas. The pilot was seriously injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot told investigators he had been practicing low approaches at a private grass airstrip and elected to land. He determined he would land too far down the runway and initiated a go-around. The engine initially responded, but the engine power then reduced to idle. He “pumped the throttle” two or three times, and the engine did not respond. He subsequently landed on the runway. The airplane overran the end of the runway and nosed over, which resulted in substantial damage to the rudder, elevator, and left wing. Postaccident engine runs were conducted with the engine attached to the airframe. The engine started on the third attempt and was allowed to idle for about 2.5 minutes. The engine remained at about 1,050 rpm during that time. Several attempts were subsequently made to increase engine rpm. The engine would sputter and lose power momentarily each time the throttle was advanced. During one engine run, the engine stopped following an attempt to advance the throttle. The engine started again on the fourth attempt. The sputtering and loss of power repeated during each attempt to advance the throttle above idle. The engine was allowed to idle and attempts to shut down the engine normally using the throttle and mixture controls were unsuccessful. The engine shut down when the ignition was turned off. The engine was equipped with a Rotech Mk.II 48-4/5 throttle body injector (TBI) unit. Examination revealed both the throttle and mixture controls were secured properly to the TBI unit. The TBI was removed from the engine and examined. No anomalies were noted during examination of the TBI that would have precluded normal operations. Functional testing of the TBI could not be completed due to a lack of access to appropriate test equipment.