3. Severe
pistoa
damage.
4,
Evidenoe of external
oil
leakage.
Sludge is
a natural by-product
of
cumbustion
and a small acrumulation is normal. Excessive sludge forma-
tion
could
indicate
several
things. The most
common
cause is
perhaps
too infrequent oil
and
oil fflter
chauges.
It
can also indicate operation
with improper
igaition
timiag or
overrich carburetor
adiustment or
a
poorly
serviced clogged air
cleaner
which restricts
air intake
and
also
results
in
an
overrich mixture.
Soring
of
the Cylinder
lVall
Unburnt fuel
not
only
adds
to
sludge forzration but can, in severe cases
cause scuffing and scoring
of the
cyl-
inder walls. As raw fuel seeps down the
cylinder
walls,
it washes
the
necessary
lubricating oils
off
the
piston
ard
cylinder
walls so
that the
piston
rings make metd to
metal
contact with the
walls.
Scoring
of
the
cylinder
walls
can also be caused by localized
hot
spots resulting from
blocked
cooling
fins
or from
inadequate or contsminated
lubrication.
Figure 23
-
Rcmove
Flywhecl
Wirh
Puller
Figure 24
-
Using Volvc Comprersor
Severe
Piston Damage
Maior damage
to
pistons
and
rings
can
take
various
forms. The top of the
piston
ring
may
be burned througb
or the
top
groove
may be
excessively
worn
and the
ring broken or stuck in the
gnxrve.
firis
can
be attributed
to
abnormal
combustion. If
ignition
timing
is overadvance4 ignition
will
occr.r while the
piston
still has
a
long
distance
to
travel on
its compression
stroke.
As a resulg the ombined hgat of compression
plus
the heat
of
pre-
igpited
fuel raises temperatures
to values
comparable to
that
of an
acetylene
torch. This,
of @urse, acts
mainly
on
tho top
land and
top ring
of
the
piston
and
results
in
early failure.
Evidence
of Extemd Oit Leakage
If
excessive
oil
leakage
is
evident,
this
may
indicate improperly
senied
breather systems. Norrrally,
an
engine
operates
internally at
pressures
under atmospheric or,
in
other words, with a
negative
crankcase
pressure.
If
lnsitive
pressures
build
up
within
the crankcase from
a
clogged
breather or'from
piston
blow-by,
oil will
be
forced
out
of an
engine at oil seals,
gaskets
or
any
other available slnt
28
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