What was found
AIRCRAFT ENGINE RUNNING ROUGH AND BACKFIRING ON FLIGHT TEST. PILOT RETURNED AND GROUNDED AIRCRAFT FOR MAINTENANCE. MAINTENANCE PERSONNEL PERFORMED INTERNAL INSPECTION OF CONVERTED MAGNETOS TO PRESSURIZATION. MAGNETO CONVERTED TO PRESSURIZED, INSPECTION REVEALED CARBON BRUSHES MISSING ON BOTH MAGNETOS, INTERNAL TIMING WAS WRONG ON ONE OF THE MAGNETOS, WIRES WERE CHAFED, EXCESS GREASE WAS FOUND ON ROTOR BRUSH ASSEMBLY. FURTHER INVESTIGATION OF AIRCRAFT WITH MAGNETO PN 10-550556-101 MODIFIED, INSTALLED ON AIRCRAFT FOR TURBO NORMALIZING. 6 ADDITIONAL MAGNETOS EXHIBITED THE SAME CONDITIONS LISTED ABOVE. RECOMMENDATIONS FOR CORRECTION ARE TO CONVERT MAGNETOS IAW PUBLISHED SPECS AND PROCEDURES. (K)