The pilot reported that while he was hovering the helicopter at about 25 feet agl, the engine suddenly lost power. He placed the helicopter in an autorotation but maintained the throttle setting that had been set while the aircraft was hovering. Just as the skids were about to contact the bushes, the engine momentarily regained power. The helicopter impacted the ground on its right hand side and fire broke out. Both the occupants escaped from the helicopter before it was destroyed by fire. An examination of the engine revealed that both valves in one cylinder had been striking the top of the piston, and the inlet valve was badly chipped. It was likely that when the inlet valve failed, a flashback occurred in the induction system, resulting in a complete loss of engine power. The engine regained power momentarily when the mixture build-up again reached a combustible level. Had the pilot closed the throttle when the engine initially failed, the sudden power surge should not have affected his ability to control the autorotation.
