As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PILOT REPORTED ON RUN UP, SELECTED LT MAGNETO POSITION, HAD EXCESSIVE RPM DROP AND BACKFIRED. ON TROUBLESHOOTING TECH FOUND MAGNETO TO BE AT FAULT. INSTALLED SERVICEABLE MAGNETO, OPS CHECKED OK. NO PROBABLE CAUSE OR RECOMMENDATIONS AT THIS TIME.