The below gauges show what can be diagnosed with a vacuum gauge, the dark red (solid) needle is the steady state reading where the light red (opaque) needle shows when the needle is fluctuating.
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Engine in good
condition will
give a reading
of 17 to 21
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When the
accelerator is
pressed the reading
will drop to below
5 then go to 25
before returning
to normal
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A reading
lower than
normal could
indicate worn
piston rings
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If the carbs
are not well
adjusted then
the reading could
drift between 12
and 16
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When the reading
drops 3 to 5 then
this could indicate
a sticking valve
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When the reading
drops by more than
7 it could indicate
a burnt valve when
that cylinder
operates
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When the reading
drops 3 to 4
it could be due to
a leaky valve
whenever that
valve operates
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When the needle
vibrates between
14 and 19 it could
indicate loose
valve guides
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A low reading of
below 5 could
indicate a leaky
manifold,
manifold gasket or
carb gasket
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When the reading
is between 14 and
8 this could indicate
incorrect valve
timing
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A reading of 13 to
16 could indicate
incorrect ignition
timing
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When the reading
drifts slowly
between 14 and
16 it could be
the plug gaps are
too close or the
contact breaker
points are not
synchronized
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Wide variations
could indicate that
there are weak or
broken valve
springs, increases
with engine
speed
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After a normal
reading which then
drops to 0 could
indicate a blocked
exhaust
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An occasional
drop as the
cylinder fires
could indicate that
this cylinders plug
is not firing or
open valve
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If normal is 20
and reading is 14
check timing as
spark could be
retarded
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Wide variations
could be a
blown head
gasket
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