Mercury Mariner 200, 225 Optimax Outboards Service Manual, 90-855348, Page 25Get this manual
Engine DETONATION
Detonation in 2-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 roller bearingsA few of the most common causes of detonation in marine 2-cycle application are as follows: aOver-advanced ignition timing(1 Use of low octane gasoline(2 Propeller pitch too high (engine RPM below recommended maximum range)(3 Lean fuel mixture at or near wide-openthrottle(4 Spark plugs (heat range too hotincorrect reachcross-firing)(5 Inadequate engine cooling (deteriorated cooling system)(6 Combustion chamberpiston deposits (result in higher compression ratio)Detonation usually can be prevented if: 1The engine is correctly set up2Diligent maintenance is applied to combat the detonation causes