Mercury Mariner Service Manual 6, 8, 9.9 210CC Sailpower, Page 11Get 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 ignitionbut 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 pistonpiston rings or piston ring lands, piston pin and roller bearings(Figure 11) While there are many causes for detonation in 2cycle engine, emphasis is placed on those causes which are most comm(:n in marine 2-cycle application fewwhich are not commonly understoodare: 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 hotincorrect reachcross-firing)6Inadequate engine cooling (deteriorated cooling system)7Combustion chamberpiston deposits (result in higher compression ratio)Detonation usually can be preventedprovided that 1) the engine is correctly set up and 2) diligent maintenance is applied to combat the detonation causes, listedpreceding