Evinrude 4906B 4HP 1969, Owner'S Manual

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Today, numerous type spark plugs are available for
service. ALWAYS check with your local marine dealer
to be sure you are purchasing the proper plug for the
engine being serviced.
Fouled: A fouled spark plug may be
caused by the wet oily deposits on the
insulator shorting the high-tension current
to ground inside the shell. The condition
may also be caused by ignition problems
which prevent a high-tension pulse to be
delivered to the spark plug.
Carbon Deposits: Heavy carbon-like de-
posits are an indication of excessive oil in
the fuel. This condition may be the result
of poor oil grade, (automotive-type instead
of a mar ine-type); improper oil-fuel mixture
in the fuel tank; or by worn piston rings.
Overheating: A dead white or gray insu-
lator, which is generally blistered, is an
indication of overheating and pre-ignition.
The electrode gap wear rate will be more
than normal and in the case of pre-ignition,
will actually
cause the electrodes to melt as
CRACK
Cut-a-way drawing showing major spark plug parts.
POLARITY CHECK 5-3
shown in this illustration. Overheating and
pre-ignition are usually caused by improper
point gap adjustment; detonation from using
too-Iow an octane rating fuel; an excessive-
ly lean air-fuel mixture; or problems in the
cooling system.
Electrode Wear: Electrode wear results
in a wide gap and if the electrode becomes
carbonized it will form a high-resistance
path for the spark to jump across. Such a
condition will cause the engine to misfire
during acceleration. If all plugs are in this
condition, it can cause an increase in fuel
consumption and very poor performance
during high-speed operation. The solution is
to replace the spark plugs with a rating in
the proper heat range and gapped to specifi-
cation.
Red rust-colored deposits on the entire
firing end of a spark plug can be caused by
water in the cylinder combustion chamber.
This can be the first evidence of water
entering the cylinders through the exhaust
manifold because of an accumulation of
scale or defective exhaust shutter. This
condition
MUST
be corrected at the first
opportunity. Refer to Chapter 3, Engine
Service.
5-3 POLARITY CHECK
Coil polarity is extremely important for
proper battery ignition system operation. If
a coil is connected with reverse polarity,
the spark plugs may demand from 30 to 4-0
percent more voltage to fire. Under such
demand conditions, in a very short time the
coil would be unable to supply enough volt-
age
to fire the plugs. Anyone of the
following three methods may be used to
quickly determine coil polarity.
1-
The polar ity of the coil can be check-
ed using an ordinary D.C. voltmeter. Con-
nect the positive lead to a good ground.
With the engine running, momentarily touch
the negative lead to a spark plug terminal.