TM 9-4910-677-14 & P
enough so the reading is 1/4 to 1/2 inch
Just as a vacuum gauge may be
below the highest steady reading.
used to adjust ignition timing, it
may also be used, in much the same
An intake system leak will also lower
manner, to adjust the idle screw in a
the reading but it may be so small
carburetor by richening the mixture
that the pointer may still be within
More
to the highest steady reading. Thus,
the so called normal range.
a faulty carburetor adjustment will
will be said about intake system
affect the vacuum reading.
leaks and timing later, but at the
moment, we wish to emphasize (1)
Valve timing too, affects vacuum
that the higher the vacuum gauge
reading, the better; provided the ig-
gauge readings. Valve timing can be
late because of improper installation
nition is not so far advanced that the
of timing gears but more common
engine pings under normal accel-
than this, tappets are set too loose
eration and (2) that an engine is not
causing the valves to open late (and
necessarily in perfect working order
T h i s condition
close too early).
just because the vacuum reading is
would reduce power considerably.
within the so called normal range on
the gauge.
Consequently, the speed of the pis-
tons would be slowed and this would
be still another reason why the vac-
Normal steadiness of the gauge
uum gauge reading might be low.
pointer varies according to the num-
ber of cylinders in the engine.
On
four cylinder engines, the intake
Speed of the pistons will be slowed
strokes do not overlap. The vacuum
by anything that reduces power, such
as shorted spark plugs, leaky igni-
gauge responds to this with an even
On
tion wires, or any of a number of
rippling motion of the pointer.
six cylinder engines, there is only
ignition difficulties which may
a faint trace of this rippling and on
ground out a cylinder. To learn the
8 cylinder engines, it is almost non-
effect of a dead cylinder, connect
existent, except on engines which
the vacuum gauge to an engine and
have overlapping valve timintg. The
short out first one, then two spark
best way to become familiar with a
plugs, etc. and study the effect. You
normal steady reading is to connect
may be surprised by the small re-
the gauge to engines known to be in
duction in speed and vacuum at idle
good operating condition and then to
speed by shorting out only one cyl-
study the readings.
inder, especially on an 8 cylinder
engine. The reduction is greater on
Responses of the gauge to move-
a 6 cylinder engine and quite notice-
ments of the throttle valve are much
able on a 4 cylinder engine.
more exaggerated in a driving test
REACTION TO LEAKAGE
than in a floor test, but in either
case, the vacuum will fall off when
It should be understandable that
the throttle is suddenly opened and
a leak in any of the passageways be-
it will rise when the throttle is sud-
tween the carburetor and the cyl-
denly closed.
In a floor test, it is
inders will interfere with the flow of
normal for the pointer to oscillate
air and fuel from the carburetor to
b e t w e e n approximately 3 and 25
the cylinders. Defects of this nature
inches as the throttle is suddenly
affect the speed of the pistons too;
opened and closed. If the vacuum
but much more directly they inter-
gauge does not respond in this way,
fere with the actual creation of the
it is an indication that leakage exists
vacuum.
A leak in the manifold
either in the form of poorly seating
gasket (depending on its size) can be
valves or in the form of poorly seal-
responsible for a reading anywhere
ing piston rings.
from slightly above zero to slightly
below normal. L e a k a g e i n t o t h e
REATION TO SPEED CHANGES
manifold has much more effect on
A low reading on the gauge may
the gauge reading than defects that
indicate late ignition timing. A high
affect speed only. A very tiny leak
unsteady reading may indicate ad-
will lower the vacuum reading con-
vanced timing:
siderably.
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