What was found
ENGINE START, RUN UP AND TAKEOFF NORMAL, WHEN THROTTLE WAS RETARDED, ENGINE BEGAN TO RUN ROUGH WITH POPPING OUT EXHAUST. MIXTURE WAS READJUSTED WITH NO CHANGE. ADVANCED THROTTLE AND RETURNED TO HOME STATION. LANDING UNEVENTFUL. ENGINE RUN UP AFTER ENGINE HAD COOLED COMPLETELY INDICATED NO DEFECTS, ALL OPERATION NORMAL. MAINTENANCE INSPECTION OF ENGINE FOUND NO DEFECTS. CONCLUDED THAT FUEL VAPORIZATION CAUSED ROUGHNESS AT IDLE IN FLIGHT WITH INCREASED TEMPS, RETARDED THROTTLE AND REDUCED AIRFLOW THRU ENG AS THIS HAS BEEN CONTINUING PROBLEM. COOLING MODIFICATIONS HAVE BEEN ACCOMPLISHED ON AC ENGINE TO INCLUDE INVERTING FUEL FLOW DIVIDER.