What happened
During the engine start and taxi phases, the pilot noted that the engine was backfiring. The pilot consulted a mechanic, who was also a passenger on the flight, regarding whether this indicated a problem; the mechanic stated it was not an issue because an engine run had just been completed.
Witnesses observing the taxi, runup, and takeoff reported hearing the engine backfiring and "snapping" from the exhaust pipe during the runup. While the aircraft appeared to have normal acceleration during the takeoff roll, the pilot noted that immediately after rotation, the aircraft failed to accelerate. After clearing a second line of trees, the pilot lowered the nose briefly in an attempt to regain airspeed, but the aircraft continued to lose both altitude and airspeed. This resulted in the left wing hitting a tree.
The investigation
An examination of the engine revealed that all twelve spark plugs were severely worn and fouled with carbon and lead deposits. Additionally, the right magneto was found to be moderately fouled with oil and contained carbon tracks within the distributor. Aircraft log books indicated that an annual inspection had been completed on the day of the accident.