1987-1993 Mercury Mariner Outboards 70/75/80/90/100/115HP 3 and 4-cylinder Factory Service Manual, Page 17Get this manual
Detonation: Causes and Prevention Detonation in 2-cycle engine somewhat resembles the "pinging" heard in an automobile engineIt can be otherwise described as tin-like "rattling" or "plinking" soundDetonation generally is thought of as spontaneous ignition, but it is best described as noisy explosion in an unburned portion of the fuelair charge after the spark plug has firedDetonation creates severe, untimely, shock waves in the engine, and these shock waves often find or create weakness: The dome of piston, piston rings or piston ring lands, piston pin and roller bearingsWhile there are many causes for detonation in 2-cycle engine, emphasis is placed on those causes which are most common in marine 2-cycle applicationA few, which are not commonly understood, are: 1Over-advanced ignition timing2Use of low octane gasoline3Propeller pitch too high (engine RPM below recommended maximum range)4Lean fuel mixture at or near wide-open-throttle5Spark plugs (heat range too hot incorrect reach cross-firing)6Inadequate engine cooling (deteriorated cooling system)7Combustion chamberpiston deposits (result in higher compression ratio)Detonation usually can be prevented, provided that 1) the engine is correctly set up and 2) diligent maintenance is applied to combat the detonation causes, listed, preceding