Evinrude 4906B 4HP 1969 Owner's Manual page 161

5-2 IGNITION
5-2 SPARK PLUG EVALUA TION
Removal: Remove the spark plug wires
by pulling and twisting on only the molded
cap. NEVER pull on the wire or the connec-
tion inside the cap may become separated
the boot damaged.
and keep them in order.
tilt the socket as you remove the plug or the
insulator may be cracked.
Examine: Line the plugs in order of
re moval and carefull y examine the m to de-
termine the firing conditions
der. If the side electrode
the center electrode,
too far upward in the cylinder and striking
the spark plug. Such damage indicates
wrist pin or the rod bearing is worn exces-
sively. In all cases, an engine overhaul
required to correct the condition.
the cause of the problem, turn the engine
over by hand. As the piston moves to the
full up position, push on the piston crown
with a screwdr iver
spark plug hole, and at the same time rock
the flywheel back-and-forth.
is
the piston detected,
rebuilt.
Correct Color:
should be dry and powdery.
inside the shell indicate
being mixed with the fuel. The most import-
ant evidence is the light gray color of the
porcelain, which is an indication
has been running at the correct
ture. This means the plug is one with the
correct heat range and also that the air-fuel
mixture is correct.
Rich Mixture:
This spark plug is foul from operating
rich condition, possibly an improper
ment.
Remove the spark plugs
TAKE CARE not to
in each cylin-
is bent down onto
the piston is traveling
To verify
inserted through
If any play in
the engine must be
A proper firing
Hard deposits
too much oil is
this plug
tempera-
A black, sooty condition
with an
carburetor adjust-
or
Damaged spark plugs.
on the left plug.
removed
before returning
on both the spark plug shell and the porce-
the
is
lain caused by an excessively rich air-fuel
mixture, both at low and high speeds.
is
rich mixture lowers the combustion temper-
ature so the spark plug does not run hot
enough to burn off the deposits.
Deposits formed only on the shell is an
indication
the
too rich.
mixture, the temperature
chamber
deposits on the insulator.
Too Cool:
plug
few deposits, indicates
too cool.
low compression or by using a spark plug of
an incorrect
shows on only one plug it
caused by low compression
If all of the plugs have this appearance,
it
is probably due to the plugs having a too-
low heat range.
over-
This spark plug has been operating too-cool,
it
is rated with a too-low heat range for the engine.
Notice the broken electrode
The broken part MUST be found and
the engine
the low-speed air-fuel
At high speeds with the correct
in the combustion
is high enough to burn off the
A dark insulator,
the plug is running
This condition can be caused by
heat range. If this condition
is most usually
in that cylinder.
to
service.
The
mixture is
with very
then
because