Conditions Affecting Operation; Detonation: Causes And Prevention; Compression Check - Mercury 45 Jet 50-55 Service Manual

Conditions Affecting Operation

1. Proper positioning of the weight inside the boat (per-
sons 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 porpoise.
(3) Can make the bow bounce excessively in
choppy water.
(4) Will increase the danger of the following -
wave splashing into the boat when coming 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).
2. Boat 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 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 com-
mon. "Rocker" exists if bottom is convex in fore-
and-aft direction when viewed from the side, and
boat has strong tendency to porpoise.
c. 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.
3. Gear Housing: If unit is left in the water, marine vege-
tation may accumulate over a period of time in certain
types of water. This growth must be removed 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 other-
wise described as a tin-link "rattling" or "pinging" sound.
90-817643R1 DECEMBER 1996
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, untimely, shock waves in the
engine, and these shock waves often find or create a weak-
ness: 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 application. 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).
Detonation usually can be prevented, provided that 1) the
engine is correctly set up and 2) diligent maintenance is
applied to combat the detonation causes, listed, preceding.
Damaged Piston Resulting from Detonation

Compression Check

1. Remove spark plugs.
2. Install compression gauge (a) in spark plug hole.
3. Hold throttle plates at W.O.T.
GENERAL INFORMATION and SPECIFICATIONS - 1-21
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