Evinrude 4906B 4HP 1969, Owner'S Manual

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2-4 TUNING
Damaged spark plugs. Notice the broken electrode
on the left plug. The broken part must be found and
removed before returning the engine
to
service.
between cylinders is far more important
than the actual readings. A variation of
more than 5 psi between cylinders indicates
the lower compression cylinder may be de-
fective. The problem may be worn, broken,
or sticking piston rings, scored pistons or
worn cylinders. These problems may only be
determined after the head has been remov-
ed. Removing the head on an outboard
engine is not that big a deal and may save
many hours of frustration and the cost of
purchasing unnecessary parts to correct a
faulty condition.
2-4 SPARK PLUG INSPECTION
Inspect each spark plug for badly worn
electrodes, glazed, broken, blistered, or lead
fouled insulators. Replace all of the plugs,
if one shows signs of excessive wear.
Make an evaluation of the cylinder per-
formance by comparing the spark condition
with those shown in Chapter 5. Check each
spark plug to be sure they are all of the
same manufacturer and have the same heat
range rating.
FOULED
ELECTRODES
A foul spark plug. The condition of this plug
indicates problems in the cylinder that should be cor-
rected.
Inspect the threads in the spark plug
opening of the head and clean the threads
before installing the plug. If the threads are
damaged, the head should be removed and
and a Hell-coil insert installed. If an at-
tempt is made to drill out the opening with
the head in place, some of the filings may
fall into the cylinder and cause damage to
the cylinder wall during operation. Because
the head is made of aluminum, the filings
cannot be removed with a magnet.
When purchasing new spark plugs, AL-
WAYS ask the marine dealer if there has
been a spark plug change for the engine
being serviced.
Crank the engine through several revo-
lutions to blowout any material which
might have become dislodged during clean-
mg.
Install the spark plugs and tighten them
to a torque value of 17 ft-lbs. ALWAYS use
a new gasket and wipe the seats in the block
clean.
The gasket must be fully compressed
on clean seats to complete the heat transfer
process and to provide a gas tight seal in the
cy Ii
nder, If the torque value is too high, the
heat will dissipate too rapidly. Conversely,
if the torque value is too low, heat will not
dissipate fast enough.
2- 5 IGNITION SYSTEM
Only one ignition system, a flywheel-
magneto, is used on outboard engines cover-
ed in this manual. If the engine perfor-
mance is less than expected, and the igni-
tion is diagnosed as the problem area, refer
to Chapter 5 for detailed service proce-
dures. To properly synchronize the ignition
system with the fuel system, see Chapter 5.
Today, numerous type spark plugs
are
available for
service.
ALWAYS
check with your local marine dealer
to
be sure you are purchasing the proper plugs for the
engine being serviced.