Mercury Mariner 4, 5, and 6HP 4-Stroke Factory Service Manual, Page 28Get this manual
GENERAL INFORMATION
DETONATION Detonation in 4-cycle engine resembles the "pinging" heard in an automobile engineIt can be otherwise described as tin-like "rattling" or "plinking" soundDetonation is an explosion of an unburned portion of the fuelair charge after the spark plug has firedDetonation 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 4-cycle application are as follows: Over-advanced ignition timingUse of low octane gasolinePropeller pitch too high (engine rpm below recommended maximum range)Lean fuel mixture at or near wide-open-throttleSpark plugs (heat range too hotincorrect reachcross-firing)Inadequate engine cooling (deteriorated cooling system)Combustion chamberpiston deposits (result in higher compression ratio)