Tech-TorqueYour technical guide to using Gunson tools

Colortune See inside the combustion chamber
Colortune See inside the combustion chamber

Colortune is a unique mixture setting device and a detector of other mixture related problems. It is a spark plug, which runs the engine like any normal spark plug, but because it has a glass insulator, it allows the colour of the combustion flame inside the cylinder to show through andindicate the strength of the mixture.

How Colortune works:
Combustion at its most efficient produces a clear blue flame, various colours result in inefficient mixtures. Inside an internal combustion engine those same colours are present though not normally seen. With Colortune fitted in place of a spark plug, the flame is visible through the glass insulator and the fuel is ignited by the spark in the Colortune.

Colortune Product Image
Colortune: Spark Plug Removal
Colortune: Fitting the Colortune Spark Plug
Colortune fits most petrol/LPG, two and four-stroke, car and motorcycle engines with single, twin or multiple carburettors or tuneable fuel injection systems. Before fitting, start the engine and warm up to normal operating temperature. Be aware of hot engine parts and remove the spark plug and fit the Colortune. Do not over tighten.
Colortune: Adaptor Lead
Colortune: Attach the Adaptor Lead
Colortune: Start Engine
Screw the adaptor lead to the centre electrode of the Colortune. Plug the other end of the adaptor lead into the plug cap on the end of the HT lead.

Now start the engine — the combustion flames will be clearly visible.

On systems which have a slow running mixture adjustment,

the position at which yellow disappears and leaves only blue, is the richest setting that should be used. (Refer to “combustion flame appearance”, below.)
Colortune: use the supplied hex adaptor to tighten the Colortune plug.
Colortune: hex adaptor fitted
Colortune: refit the spark lead.
The same procedure is used for cars. Multi-cylinder engines can be tested with a Colortune in each cylinder however a single unit will enable the mixture to be observed
one cylinder at a time.

The rubber bush in a spark plug spanner will not retain the Colortune as the ceramic is much smaller. A black hex adaptor is

supplied to tighten the Colortuneusing a standard 14mm plug socket. Care should be taken not to bend the centre electrode when fitting the adaptor lead.
Colortune: Use supplied Viewscope
Colortune: lower part of viewscope can be used to tighten.
Colortune: position highly polished sight flap.
A viewscope is provided to assist when a clear view of the Colortuneis not possible in a crowded engine bay. The lower half of the viewscope can be used to hand
tighten the Colortune when the plug seat is recessed. The adaptor lead runs through the centre of the viewscope and emerges through the slot in the upper half.
Position the highly polished sight flap to get a clear view of the combustion flames. On this SU carburettor a cap is removed to access the mixture adjustment.

Simple engine tests with Colortune:

Part throttle: If the throttle is opened very slowly the blue flame should become slightly lighter — this is due to a slightly leaner mixture designed to give improved economy at part throttle operation.

Full throttle: When there is a sudden demand for power a richer mixture will prevent a “flat spot”. The blue flame will change to yellow when the throttle is opened.

*Full and detailed instructions are provided with every Colortune kit.


The general appearance of the combustion flame when viewed through Colortune is as follows:
Yellow: Indicates a fuel/air mixture which has excessive petrol content(too rich).
Bunsen Blue: Correctly proportioned mixture.
Whitish Blue: Less petrol content(too lean) - at idle, misfiring and engine instability may occur before this lean mixture is fully visible.