Engine Compression - Mercury Optimax 115 Service Manual

GENERAL INFORMATION
Engine
DETONATION
Detonation in a 2-cycle engine resembles the "pinging" heard in an automobile engine.
It can be otherwise described as a tin-like "rattling" or "plinking" sound.
Detonation is an explosion of an unburned portion of the fuel/air charge after the spark
plug has fired. Detonation creates severe shock waves in the engine, and these shock
waves often find or create a weakness: The dome of a piston, cylinder head/gasket, piston
rings or piston ring lands, piston pin and roller bearings.
A few of the most common causes of detonation in a marine 2-cycle application are as
follows:
Over-advanced ignition timing.
Use of low octane gasoline.
Propeller pitch too high (engine RPM below recommended maximum range).
Lean fuel mixture at or near wide-open-throttle.
Spark plugs (heat range too hot – incorrect reach – cross-firing).
Inadequate engine cooling (deteriorated cooling system).
Detonation usually can be prevented if:
1. The engine is correctly set up.
2. Diligent maintenance is applied to combat the detonation causes.

Engine Compression

Engine compression should be checked with engine block warm, throttle shutter wide
open, all spark plugs removed and using a fully charged battery. Normal compression for
all cylinders should be 110 to 130 psi (758.5 to 896.4 kPa). Cylinders should not vary more
than 15 psi (103.4 kPa) between one another. A variance of more than 15 psi would indi-
cate the need for a power head inspection/disassembly.
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90-855347R1 JANUARY 1999