Tuning and fault finding with a vacuum gauge


Vacuum gauges are a cheap yet effective method of ensuring that your classic is well tuned and problem free. The vacuum gauge is a useful instrument to have on any classic as it can be used to tune for power, diagnose engine faults as well as helping to increase and maintain fuel economy.

Whether the gauge you choose is a test gauge or a permanent dash mounted instrument it will need to be connected to the car's inlet manifold. Often there is a connection or pipe already but if not it may be necessary to drill and tap the manifold.

Once your gauge is connected and seen to be registering a vacuum you can begin the tuning process

(Normal vacuum at idle should be about 18-22In (4 Cyl), 19-21In (6 Cyl) or 15-18In on low compression engines ('In' is an abbreviation for inches of mercury))

  • Start by first warming the engine and note the idle vacuum reading

  • Adjust the mixture on your carburetor(s) to obtain the highest steady vacuum reading.

  • Richen the mixture setting until the needle begins to fall. This will give you the correct mixture setting for maximum power.

  • Slacken the distributor clamp bolt, and with the engine still at idle advance or retard the ignition until the highest steady vacuum reading is obtained.
  • Retard the timing until the vacuum gauge reading drops slightly (½In).

Your engine should now be tuned to perfection. Tuning with the engine running compensates for wear in the timing gear and therefore will provide better results than using the manufacturers ignition settings.

Fault Finding

Use this hand chart to diagnose engine problems quikly and easily.


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 by 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 suggest a
blown head