As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CRUISE POWER SETTING IN FLIGHT. RT ENGINE RPM FLUX 100-200 RPM AND MANIFOLD PRESSURE FLUX 1 INCH TO 2 INCH. DURING GROUND RUN TO TROUBLESHOOT SYSTEM ENGINE RPM WOULD SUDDENLY DROP 200-400 RPM. AT CRUISE SETTING. PERFORMED MAGNETO CHECK. LT MAG FAILED. BEARING IN DISTRIBUTOR BLOCK SPINNING, POSSIBLE DATE CODE ON BLOCK 01/09.