Event
A commercial certificated pilot was conducting a VFR cross-country positioning flight to his home airport. While cruising approximately 500 feet above the ground, the engine experienced three to four surges. The pilot engaged the electric fuel boost pump and switched fuel tanks, despite both tanks containing adequate fuel. The engine subsequently stopped, prompting the pilot to select an emergency landing area in soft, marshy terrain. The landing resulted in damage to the nose gear, engine firewall, left and right landing gear, and right wing.
Investigation
Postaccident examination of the aircraft revealed that the engine had accumulated approximately 500 hours since its last overhaul. Each fuel tank contained fuel, and the electric fuel pump produced pressure. Opening the throttle yielded only a momentary increase in fuel pressure. After several attempts, the engine started and ran at idle. Maintenance records showed the engine-driven fuel pump had been replaced twice since the overhaul. Each replacement pump was tested and produced proper fuel pressure.
The engine fuel servo was removed and tested for operation. During fuel flow testing, the servo produced a lean fuel mixture at a mid-range power setting. Following the flow check, the servo was disassembled. Examination disclosed that the brass idle plug/main metering valve had part number 2541461 stamped on opposite sides of the valve's vertical surface. One of these part numbers had been obliterated with a marking tool, and the number 2537665 was etched onto the valve by hand. Part number 2537665 is the correct component for the servo model. The valve appeared to be a hand-modified version of the 2541461 part, which has a different metering jet placement than the correct 2537665 valve. The modification was accomplished by several punch tool indentations around the edges of the jet hole on both sides of the valve body, and these indentations had hanging burrs protruding into the jet hole.
Findings
The examination identified that the fuel servo's idle plug/main metering valve had been altered by hand, resulting in burrs projecting into the metering jet hole. This modification contributed to a lean fuel mixture condition at mid-range power settings, consistent with the engine failure experienced during flight.