What happened
A commercial certificated pilot was conducting a VFR cross-country positioning flight to his home airport. While the aircraft was in cruise flight at approximately 500 feet above the ground, the engine experienced three to four surges. In response, the pilot activated the electric fuel boost pump and switched fuel tanks, despite both tanks containing adequate fuel levels.
The engine subsequently quit, prompting the pilot to select an emergency landing area consisting of soft, marshy terrain. The aircraft sustained damage to the right wing, the engine firewall, the nose gear, the left landing gear, and the right landing gear. The pilot was not injured.
The investigation
A post-accident examination of the engine showed it had flown approximately 500 hours since its last overhaul. Investigators confirmed that both fuel tanks contained fuel and that the electric fuel pump produced pressure. However, opening the throttle resulted in only a momentary increase in fuel pressure. After multiple attempts, the engine restarted and ran at an idle.
Maintenance records indicated the engine-driven fuel pump had been replaced twice since the overhaul, with each replacement being tested for proper operation and producing sufficient pressure.
Upon inspection of the engine fuel servo, testing revealed it produced a lean fuel mixture at a mid-range power setting. Disassembly of the servo, which bore an overhaul facility's lead seal, revealed that the brass idle plug/main metering valve had been hand modified. The valve displayed two different part numbers: 2541461 and 2537665 (the latter being the correct part for that servo model). One number had been obliterated by a marking tool, while the other was etched by hand. The investigation determined the component was a modified version of part number 2541461, which features different metering jet placement than the correct part. This modification involved using a punch tool around the edges of the jet hole, leaving hanging burrs protruding into the hole.