What was found
WHILE REDUCING POWER ON LANDING, ENG RAN VERY ROUGH. AFTER LANDING MAGNETO CHECK CAUSE LARGE DROP IN RPMS ON BOTH MAGS. MECHANICS FIRST REPAIRED INDUCTION LEAK WHICH DID NOT SOLVE MAG DROP. THEN THEY CHANGED THE CARBURETOR WHICH DID NOT SOLVE PROBLEM, THEN THEY SENT BOTH MAGNETOS IN FOR REBUILDING. THEY CHANGED A LOT OF PARTS IN THE MAGS AND THEN PLANE OPS SATISFACTORILY. THIS DUAL MAGNETO FAILURE IS A SAFETY OF FLIGHT ISSUE. THESE MAGS WERE REBUILT APPROXIMATELY 400 HRS BEFORE THEIR FAILURE BY AN FAA APPROVED REPAIR STATION. THERE IS A REQUIREMENT THAT THEY BE INSPECTED EVERY 500 HS FOR CERTAIN OPERATORS. THIS INCORRECTLY IMPLIES THAT THEY SHOULD PROVIDE SAFE SERVICE UNTIL THE 500 HR INSPECTION IS DUE. WHEN TWO MAGS FAIL AFTER 400 HRS OF SERVICE, THIS 500 HR INSPECTION SHOULD BE REDUCED TO PERHAPS 300 HOURS OF OPS & SERVICE BULLETINS & AIRWORTHINESS DIRECTIVES ISSUED.