Mariner 30 JET 40 (4 CYL), Service Manual

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90-814676R1 DECEMBER 1996 IMPORTANT INFORMATION - 1-15
Conditions Affecting
Operation
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
back-and-forth (bow steer).
2. Boat Bottom: For maximum speed, a boat bot-
tom should be nearly a flat plane where it con-
tacts the water and particularly straight and
smooth in fore-and-aft direction.
a. 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 reducing boat speed. “Hook” frequently
is caused by supporting boat too far ahead of
transom while hauling on a trailer or during
storage.
b. 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 por-
poise.
c. Surface Roughness: Moss, barnacles, etc.,
on boat or corrosion of outboard’s gear hous-
ing increase skin friction and cause speed
loss. Clean surfaces when necessary.
3. Gear Housing: If unit is left in the water, marine
vegetation may accumulate over a period of time
in certain types of water. This growth must be re-
moved from unit before operation, as it may clog
the water inlet holes in the gear housing and
cause the engine to overheat.
Detonation: Causes and
Prevention
Detonation in a 2-cycle engine somewhat resembles
the “pinging” heard in an automobile engine. It can be
otherwise described as a tin-like “rattling” or “plank-
ing” sound.
Detonation generally is thought of as spontaneous
ignition, but it is best described as a noisy explosion
in an unburned portion of the fuel/air charge after the
spark plug has fired. Detonation creates severe, un-
timely shock waves in the engine, and these shock
waves often find or create a weakness: The dome of
a piston, piston rings or piston ring lands, piston pin
and roller bearings.
While there are many causes for detonation in a
2-cycle engine, emphasis is placed on those causes
which are most common in marine 2-cycle applica-
tion. A few, which are not commonly understood, are:
1. Over-advanced ignition timing.
2. Use of low octane gasoline.
3. Propeller pitch too high (engine RPM below rec-
ommended maximum range).
4. Lean fuel mixture at or near wide-open-throttle.
5. Spark plugs (heat range too hot - incorrect reach
- cross-firing).
6. Inadequate engine cooling (deteriorated cooling
system).
7. Combustion chamber/piston deposits (result in
higher compression ratio).