Mercury Mariner 4 and 5HP 4-Stroke Outboards Service Shop Manual 1999, Page 27Get this manual
GENERAL INFORMATION
DETONATION Detonation in engine resembles the "pinging" heard in an automobile engineIt can be otherwise described as tinlike "rattling" or "plinking" soundDetonation is an explosion of an unburned portion of the fuelair charge after the spark plug has fired Detonation creates severe shock waves in the engine, and these shock waves often find or create weakness: The dome of piston, cylinder headgasket, piston rings or piston ring lands, piston pin and bearingsA few of the most common causes of detonation in marine 4cycle application are as follows: Overadvanced ignition timingUse of low octane gasolinePropeller pitch too high (engine RPM below recommended maximum range)Lean fuel mixture at or near wideopen-throttleSpark plugs (heat range too hotincorrect reach crossfiring)Inadequate engine cooling (deteriorated cooli ng system)Combustion chamberpiston deposits (result in higher compression ratio)