What was found
DURING A PREFLIGHT RUN, WHEN THE RT ENG CARB HEAT WAS SELECTED ON, THE ENG RPM DROPPED OVER 600 RPM & THE ENG RAN ROUGH. INSPECTION REVEALED THE NR 4 CYLINDER EXHAUST RISER HAD FRACTURED ABOUT THE FLANGE & HAD FALLEN OPEN, ALLOWING EXHAUST GASSES TO BE EMITTED FREELY INTO THE ENG COWLING & INTO THE ENG CARB HEAT AIR INLET, CAUSING THE RPM DROP & ROUGHNESS.