Mercury 225 EFI, Руководство по обслуживанию

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GENERAL INFORMATION
90-884294 OCTOBER 2001 Page 1C-5
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.
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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 90 – 110 psi (616.3 – 753.5 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.