Mercury Mariner 30/40 4-Stroke Outboard Service Manual 1998, Page 33Get this manual
GENERAL INFORMATION
CAVITATION
Cavitation is caused by water vapor bubbles forming either from sharp edge or angle on the gear case or from an irregularity in the propeller blade itselfThese vapor bubbles flow back and collapse when striking the surface of the propeller blade resulting in the erosion of the propeller blade surfaceIf allowed to continue, eventual blade failure (breakage) will occur
DETONATION
Detonation in 4-cycle engine resembles the "p inging" 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 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)