TM 9-4910-677-14 & P
4. It the pointer is unsteady and if
which applies to all V-8 engines re-
the sweeps of the pointer increase
gardless of which cylinder the manu-
with increases in speed, the trouble
facturer has designated as number
is weak or broken valve springs. If
one. On V-8 engines, ground out all
the sweeps get smaller but more
the other cylinders and run on the
rapid on increasing the speed, the
following pairs.
trouble is in intake system leaks. If
Run on cyl. A and F Run on cyl. D and G
the pointer steadies on increasing
speed, the trouble is ignition, faulty
Run on cyl. C and E
Run on cyl. Band H
distributor weights, or carburetion.
5. This is a normal reaction when
the throttle valve is opened and
If the pointer does not re-
closed.
spond with wide sweeps, it is an in-
dication that leakage exists. If the
p o s s i b i l i t i e s of leakage through
poor1y seating valves, manifolds,
gaskets, etc. have been exhausted,
1. When the pointer is low but
then the piston rings are not seal-
steady, the trouble is something
ing properly.
w h i c h affects all cylinders alike.
This could be late ignition timing,
6. (1) When the distributor is ad-
late valve timing, or loose tappets.
vanced to the highest steady reading
Such troubles can be responsible for
and then retarded so the pointer is
the reading being anywhere from1/4
1/4 to1/2 inch below the highest steady
to 2 inches low. Intake system leaks
reading, and (2) if the pointer is
(warped intake manifold, leaky
w i t h i n specifications and holding
manifold gasket, leaky carburetor
steady after making this adjustment
flange gasket, poorly seating intake
and (3) if the ignition is in time after
valves and leaky intake guides) can
checking with a timing light after
be responsible for the reading being
making this adjustment, the engine
anywhere from 1/4 to 16 inches low
can be considered to be in time.
depending on the size of the leak.
Allow one inch less for each 1000
feet of elevation.
2. When the pointer pulses regu-
larly, the trouble is something which
NORMAL REACTIONS
affects one particular cylinder con-
sistently; for instance, one exhaust,
18 to 22 inches is usually referred
valve which does not seat, one ex-
to as the normal range for a warmed
haust tappet which is too tight, or
up engine at idle speed, but with
one sticky valve, or one dead spark
greater and greater degrees of valve
plug. The pulses of the pointer are
o v e r l a p , this range is becoming
much greater if they are caused by
Some car manufac-
meaningless.
some form of leakage such as ex-
turers are now specifying 15 inches
haust valve not seating, rather than
as a noamal vacuum reading during
something which, only slows the pis-
engine idle. The specifications for
tons such as a dead spark plug.
the car under test should be checked.
If with proper tune-up a particular
3. An unsteady pointer may be
engine could be adjusted to 21 inches
caused by defects occuring irregu-
of vacuum, it would be considerably
larly and in several or all of the cyl-
out of time if the vacuum were ad-
inders. O t h e r defects which are
justed to only 19 inches; despite the
responsible for an unsteady pointer
fact that 19 is within the so called
are
loose
distributor
governor
normal range.
springs, rich carburetor mixture,
extremely
advanced
or
retarded
To adjust ignition
timing with a
spark, wide spark plug gaps, igni-
vacuum gauge, loosen
the distributor
tion points not synchronized, or in-
setscrew and advance
to the highest
take valves shifting on their seats.
STEADY
reading.
Then
retard
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