What was found
CUSTOMER REPORTED ENGINE 'BACKFIRING' INTERMITTENTLY AT HIGH IDLE. VALVE TRAIN PROBLEMS SUSPECTED & COLD CYLINDER TESTER DISCLOSED NR 3 CYL SLIGHTLY COOL (COMPRESSION WAS EXCELLENT). CLOSER INSP DISCLOSED INCORRECT SPRINGS PN 65427 INSTALLED INSTEAD OF LW-11800. NO RECORD OF MX ON THIS CYL SINCE O/H. PN 65427 SPRING HAS ABOUT 35 POUNDS LESS LOAD THAN CORRECT SPRING & IS USED IN O-235/O-290 SERIES ENGINES. VALVE & GUIDE WEAR WERE CHECKED & WITHIN LIMITS, BUT ENOUGH MUST HAVE CHANGED SINCE ENG HAS RUN 1620 HRS SMOH W/O COMPLAINT OF BACKFIRING. IN THIS CASE, 'WEAK' SPRINGS ALLOWED VALVES TO RESONATE OR BOUNCE IN A NARROW RPM RANGE (1200-1500 RPM) & RESULTED IN 'AFTERFIRING'. REPLACING SPRINGS CURED PROBLEM.