TM 9-4910-677-14 & P
(Leakage leans the mixture. When it is
fairly wide sweeps. Other defects
excessive, it burns the valves and spark
which DO NOT affect all cylinders
plugs prematurely. Even in its mildest
equally are a worn exhaust valve
form, it causes an engine to ping. It
guide, a piston ring that does not
should be noted that compensation can be
seal, a leaky spark plug gasket, etc.
made in mild cases of leakage by re-
tarding the spark. When this is done, the
engine will not develop quite as much
UNSTEADY POINTER
power or have quite as good mileage as
when the leakage is corrected and the
I f the pointer moves in wide
timing advanced to where it should be.
sweeps of 3 or 4 inches, the trouble
When adjusting the ignition timing with
could be a blown head gasket between
a vacuum gauge as explained previously,
two cylinders or ignition cross firing
leakage is automatically compensated
because of insulation breaking down
for. Or looking at it another way; if the
vacuum gauge setting is later than the
either in the distributor or in the
timing light setting it usually indicates
s p a r k p l u g c a b l e s . The trouble
that intake system leakage exists. There
could also be weak valve springs.
are other reasons, however, why it is
To determine whether or not the
sometimes necessary to retard the spark
trouble is due to cross firing, one
from the timing light setting to prevent a
plug at a time may be shorted out to
ping. Sometimes it is because of low
determine which is firing its cyl-
octane gasoline or because of com-
pression being too high).
inder at the wrong time. The sweep-
ingaction will stop when the offend-
ing plug is shorted out.
Intake system leakage is not only
found in the form of a leaky manifold
Distributor
governor
weight
gasket. It might be in the form of a
movements
are
sometimes
very
warped or loos e intake manifold,
critical at low engine speeds because
leaky carburetor flange gasket, worn
there is no spring tension on them;
throttle shaft, one or more poorly
and often they cause a continual but
seating intake valves (for any rea-
very slight change in engine speed by
son including a tight tappet), one or
moving in and out. This action causes
more worn intake valve guides, or
a continual advance and retard of the
leaks in any of the lines or fittings
spark, in turn, causing the pointer
to any vacuum operated units such as
on the gauge to be unsteady. The
windshield wipers, etc. All of these
pointer may react much the same
defects affect all cylinders equally
way if the spark is advanced beyond.
so all of them cause the vacuum
the highest steady reading.
gauge to give a low but steady read-
ing. How low the reading depends
An unsteady action of the pointer
on the size of the leak.
may also be caused by unevenly or
widely spaced spark plug gaps or by
There are other types of leakage
spark plugs that are carbonized from
which DO NOT affect all cylinders
a rich mixture or from oil; or just
alike and therefore DO NOT give a
because they are too cold for the
steady reading on the vacuum gauge.
engine. An unsteady pointer may al-
A poorly seating exhaust valve, for
so be the result of small inconsistent
instance, would affect the intake only
for one cylinder. Every time the in-
intake system leaks, such as intake
valves shifting on their seats be-
take valve for that cylinder would
cause of worn guides or such as a
open (on the intake stroke of the pis-
worn throttle shaft in a carburetor.
ton) exhaust gas would be drawn into
that cylinder. This momentary re-
Ignition points that are not synchron-
ized, insufficient tappet clearance,
duction in vacuum would react on the
weak valve springs and sticky valves
gauge as a large pulse of 2 to 4 inches
also cause the pointer of the gauge to
and much larger than the pulses
be unsteady.
caused by shorting out a cylinder.
Of course, the size of the pulse de-
Often the trouble may be isolated
pends on the size of the leak and
by adjustment of the timing and the
where more than ore cylinder is con-
carburetor and a check of the ignition
c e r n e d , the pulses change to an
circuit including inspection of the
erratic wandering of the pointer in
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