Mercury 100 FourStroke, Service Manual

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90-830234R3 DECEMBER 1997 1C-2 - IMPORTANT INFORMATION
NOVEMB
Boat
WEIGHT DISTRIBUTION
1. Proper positioning of the weight inside the boat
(persons and gear) has a significant effect on the
boat’s performance, for example:
a. Shifting weight to the rear (stern)
(1.) Generally increases top speed.
(2.) If in excess, can cause the boat to por-
poise.
(3.) Can make the bow bounce excessively in
choppy water.
(4.) Will increase the danger of the following
- wave splashing into the boat when com-
ing off plane.
b. Shifting weight to the front (bow)
(1.) Improves ease of planing off.
(2.) Generally improves rough water ride.
(3.) If excessive, can make the boat veer left
and right (bow steer).
BOTTOM
For maximum speed, a boat bottom should be
nearly a flat plane where it contacts the water and
particularly straight and smooth in fore-and-aft di-
rection.
1. Hook: Exists when bottom is concave in fore-
and-aft direction when viewed from the side.
When boat is planing, “hook” causes more lift on
bottom near transom and allows bow to drop,
thus greatly increasing wetted surface and re-
ducing boat speed. “Hook” frequently is caused
by supporting boat too far ahead of transom while
hauling on a trailer or during storage.
2. Rocker: The reverse of hook and much less
common. “Rocker” exists if bottom is convex in
fore-and-aft direction when viewed from the side,
and boat has strong tendency to porpoise.
3. Surface Roughness: Moss, barnacles, etc., on
boat or corrosion of outboard’s gear housing in-
crease skin friction and cause speed loss. Clean
surfaces when necessary.
WATER ABSORPTION
It is imperative that all through hull fasteners be
coated with a quality marine sealer at time of installa-
tion. Water intrusion into the transom core and/or in-
ner hull will result in additional boat weight (reduced
boat performance), hull decay and eventual structur-
al failure.
CAVITATION
Cavitation is caused by water vapor bubbles forming
either from a sharp edge or angle on the gear case
or from an irregularity in the propeller blade itself.
These vapor bubbles flow back and collapse when
striking the surface of the propeller blade resulting in
the erosion of the propeller blade surface. If allowed
to continue, eventual blade failure (breakage) will
occur.
Engine
DETONATION
Detonation in a 2-cycle engine resembles the “ping-
ing” heard in an automobile engine. It can be other-
wise 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. Det-
onation creates severe shock waves in the engine,
and these shock waves often find or create a weak-
ness: 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 rec-
ommended 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).
•
Combustion chamber/piston deposits (result in
higher compression ratio).