Accident Sequence
The pilot had been flying in the local area for approximately 20 minutes before entering the VFR traffic pattern. While turning onto base leg, the aircraft's engine suddenly lost all power. The pilot executed a power-off landing in an open field. During the landing, the nose gear encountered soft terrain and collapsed.
Post-Accident Inspection
A post-accident inspection determined that there was sufficient uncontaminated 100LL fuel on board. During a series of test runs, the engine ran extremely rough at RPM below 1,500 and quit several times during the run. The engine smoothed out somewhat when the fuel boost pump was turned to the low position, and smoothed out further when the pump was set to the high position.
Fuel System Tests
The un-metered fuel pressure was measured and found to be lower than the specification directed, but not low enough to account for the very rough running engine. After the test runs, the spark plugs were removed; their white ashy color confirmed that the engine was running extremely lean. Further flow tests and inspections were performed on the fuel system components after they were removed from the aircraft. The results of these tests did not reveal any anomalies that would be expected to cause the engine to run rough or quit while on a reduced power approach. Additionally, the fuel pump was tested to ensure no air was leaking past the shaft seal; the result of that test was negative.
Undetermined Cause
At the completion of the testing and inspection process, it could not be clearly determined what was causing the engine's fuel system to run so lean.