What was found
PILOT REPORTED SUDDEN ENGINE ROUGHNESS AND POWER LOSS. UNABLE TO MAINTAIN ALTITUDE, AN UNEVENTFUL ON-AIRPORT LANDING WAS MADE. INSPECTION REVEALED HEATER/MUFFLER HAD BEEN REPAIRED, BUT INSTEAD OF A 'FLAME CONE' BEING WELDED IN, A FLAME TUBE WITH SOLID END PLATE HAD BEEN INSTALLED. THIS TUBE HAD BROKEN FREE, AND UPON SLIDING AFT, THE SOLID END PLATE BLOCKED OFF THE MUFFLER OUTLET.