Fault Diagnostics with a vacuum gauge

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.

Engine in good
condition will
give a reading
of 17 to 21

When the
accelerator is
pressed the reading
will drop to below
5 then go to 25
before returning
to normal

A reading
lower than
normal could
indicate worn
piston rings

If the carbs
are not well
adjusted then
the reading could
drift between 12
and 16
When the reading
drops 3 to 5 then
this could indicate
a sticking valve

When the reading
drops by more than
7 it could indicate
a burnt valve when
that cylinder

When the reading
drops 3 to 4
it could be due to
a leaky valve
whenever that
valve operates
When the needle
vibrates between
14 and 19 it could
indicate loose
valve guides
A low reading of
below 5 could
indicate a leaky
manifold gasket or
carb gasket
When the reading
is between 14 and
8 this could indicate
incorrect valve
A reading of 13 to
16 could indicate
incorrect ignition
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
Wide variations
could indicate that
there are weak or
broken valve
springs, increases
with engine
After a normal
reading which then
drops to 0 could
indicate a blocked

An occasional
drop as the
cylinder fires
could indicate that
this cylinders plug
is not firing or
open valve

If normal is 20
and reading is 14
check timing as
spark could be

Wide variations
could be a
blown head




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| Updated 06-Feb-2008 |