Although some experienced mechanics can tune a car's
by ear, to do the job really accurately you need the help of special
While most garages use an exhaust
(also called a CO meter), a
cheap alternative is to buy a special see-through
such as the Gunson
The Colortune works on the principle that the colour of the flame produced
/air mixture igniting in the
varies with the richness of
plug is screwed into the spark plug hole in place of a
normal plug. With the
running, you can then view what's going on in the
If the fuel/air mixture is a deep blue colour, then the mixture is correct.
An orange flame means that the mixture is too rich, while a pale blue or white
flame indicates a very weak fuel/air mixture.
On cars with a single carburettor (or one twin-choke carburettor) you need
one plug, but if your car has twin carburettors you need two plugs - one for
each carb. Also note that the Colortune is made to
cars with a 14mm spark
plug. For those cars using a different size plug, you need a special
To do a really thorough job, it's a good idea to fit the Colortune plug
to each cylinder in turn. You can then see whether a particular cylinder is
Fitting the plug
Run the engine until it is warmed up, then
off. Remove the spark plug
from the engine (any one will do) and screw the Colortune plug into the spark
plug hole. Make sure the copper
ing washer is in place or the plug won't
seal properly. Then fit the spanner adapter collar over the plug and tighten up
the plug as you would for an ordinary spark plug. But don't tighten it up more
than 1/46 of a turn or you will damage the washer.
If your engine needs an adapter piece, screw it into the spark plug hole
first, then screw the Colortune plug into that.
If the spark plug hole is awkwardly placed, or you are tuning in bright
sunlight, it may be difficult to see the window in the plug. A special tube
with an angled mirror is supplied in the kit to make viewing easier. Fit the
lower half of the tube, then attach the HT lead adapter to the threaded stub in
the centre of the Colortune plug.
Now fit the top half of the tube with the mirror on it so that the HT
adapter slips through the slot in the side. Push The end of the adapter into
the vacant HT lead - make sure it is firmly clipped into the end of the
Tune the engine
Restart the engine and let it idle. Now check the viewing window to compare
the colour inside the chamber with that shown in the illustration below. If the
colour is not a deep blue, carefully adjust the carburettor mixture screw one
way or the other until the colour is the correct shade.
If you are tuning twin carburettors, adjust the mixture screw on each carb
so that the colour is the same through each plug.
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resembles the simpler variable-jet type (See
How variable-jet carburettors work
) in having a
- a constricted neck - through which air flows on its way to the
In an ideal, 100 per cent efficient
internal combustion engine
would burn to give just
and water vapour. In practice, of
are far from efficient and the
oxides of nitrogen
, as well
dioxide and water vapour.
Almost all car
work on the four-stroke
, so called because it
and exhaust -
to produce one firing of the
/air mixture. This means that the
rotates twice to complete each cycle.