A WITNESS STATED THAT THE AIRPLANE BACKFIRED WHILE IN A TURN AT LOW ALTITUDE. HE HEARD IT BACKFIRE TWICE MORE AND NOTICED ENGINE RUNNING AT IDLE JUST BEFORE IT CRASHED. WHEN RUNUP, THE ENGINE WOULD ATTAIN 1900 RPM, THEN BACKFIRE AND THE RPM WOULD DROP TO 600 RPM. WHEN THE MAGNETOS WERE REPLACED, THE ENGINE RAN WITHOUT PROBLEMS. TEARDOWN OF MAGNETOS FOUND BOTH DISTRIBUTOR BLOCK ASSEMBLIES WERE EXCESSIVELY CORRODED AND TWO CONTACTS IN EACH WERE MISSING. BOTH BREAKER POINTS WERE WORN BEYOND LIMITS.

The National Transportation Safety Board determines the probable cause(s) of this accident as follows:

Contributing Factors

MAINTENANCE,ANNUAL INSPECTION..INADEQUATE..OTHER MAINTENANCE PERSONNEL

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