As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
MAGNETO HAD A 250 RPM DROP DURING PRE-FLIGHT RUN UP. A CHECK OF THE TIMING OF MAGNETO TO ENGINE SHOWED IT TO BE APPROX 10 DEGREES RETARDED. MAG WAS NOT FOUND LOOSE AT ITS MOUNT, SO IT WAS REMOVED FOR INSPECTION. UPON DISASSEMBLY, DISCOVERED A CRACK ON THE ''EAR'' OF THE SLOT IN THE ROTOR SHAFT THAT HOLDS THE CAM FOR OPENING THE POINTS. NO RUST OR OTHER OUTSIDE FACTOR WAS VISIBLE AS A CAUSE FOR THIS FAILURE. (X)