CAVITATION
Cavitation occurs when water flow cannot follow the contour of a fast‑moving
underwater object, such as a gear housing or a propeller. Cavitation increases
propeller speed while reducing boat speed. Cavitation can seriously erode the
surface of the gear housing or the propeller. Common causes of cavitation are:
• Weeds or other debris snagged on the propeller
• Bent propeller blade
• Raised burrs or sharp edges on the propeller
ELEVATION AND CLIMATE
Elevation and climate changes will affect the performance of your power
package. Loss of performance can be caused by:
• Higher elevations
• Higher temperatures
• Low barometric pressures
• High humidity
For you to have optimum engine performance under changing weather
conditions, it is essential that the engine be propped to allow the engine to
operate at or near the top end of the specified maximum RPM range with a
normal boat load during your normal boating weather conditions.
In most cases, recommended RPM can be achieved by changing to a lower
pitch propeller.
DETONATION
Detonation in a 4‑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 the explosion of the unburned fuel/air charge after the spark plug
has fired. Detonation creates severe shock waves in the engine. These shock
waves often find or create a weakness: the dome of a piston, cylinder head or
gasket, piston rings or piston ring lands, piston pin, and roller bearings.
A few of the most common causes of detonation in a marine 4‑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
• Deteriorated or inadequate engine cooling system
• Combustion chamber deposits: result in higher compression ratio
Detonation usually can be prevented if:
• The engine is correctly set up
GENERAL INFORMATION
20 eng
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