What was found
PILOT REPORTED SMELL IN COCKPIT. NEXT TAKEOFF, HEARD LOUD RATTLE, LANDED. FOUND RT OUTER MUFFLER SHROUD LOOSE, THEN FOUND MUFFLER CRACKED AT 100.3 HRS. LT MUFFLER CHANGED. FACTORY ADVISED MATERIAL OF MUFFLER HAD BEEN CHANGED; SO, THAT 500 HRS SEEMED TO BE THE LIFE OF THESE UNITS AND STATED PROBLEM WAS POSSIBLE DUE TO SHORT HEADER STACKS AND WHEN PILOT PULLED THROTTLE BACK FAST, CAUSED BACKFIRING IN MUFFLER AND DAMAGE. (X)