Mercury Mariner V-250 V-275 Outboard Service Shop Manual 1990, Page 24Get this manual
Detonation: Causes and Prevention Detonation in 2-cycle engine somewhat resembles thepinging" heard in an automobile engineIt can be otherwise described as tin-like "rattl ing" or "plinking" sound Detonation generally is thought of as spontaneous ignitionbut it is best described as noisy explosion in an unburned portion of the fuell ai cha rge after the
3Propeller pitch too high (engine RPM below recommended maximum range)4Lean fuel mixture at or near wlde-open-throtlle5Spark plugs (heat range too hot-Incorrect reachcross-firing)6Inadequate engine cooling (deteriorated coolIng system)7Combustion chamber piston deposits (result In higher compression ratio)
spa rk plug has fired Detonation creates severe, untimely shock waves in the engi ne, and these shock waves often find or create weakness: The dome of pistonthe cylinder head gasketpiston rings or piston ring landsWhile there are man causes for detonation in 2-cycle engine, emphasis is placed on those causes which are most common in marine 2-cyc le applicationA fe w, which are not commonl understood, are: