The Science; The Practical Details - Gunson COLORTUNE G4074 Mode D'emploi

There are sometimes exceptions to the above as petrol engines are imperfect machines,
these will be described later.
• The correctly proportioned mixture is - 14.7 parts of air by weight to 1 part of
petrol (hydrocarbon fuel).
• If there is perfect combustion, all the fuel will be burnt to produce carbon dioxide
and water with no carbon monoxide or unburnt fuel (hydrocarbons).
• The carbon in the fuel burns with oxygen in the air to produce carbon monoxide
(CO), which then burns with more oxygen to produce carbon dioxide (CO2)
• Hydrogen in the fuel burns with the oxygen in the air to produce water (H2O)
• Nitrogen in the air passes through to the exhaust with little reaction.
• When there is less air in the mixture, there is not enough oxygen to complete the
burning process so some carbon monoxide is not changed to carbon dioxide and
hydrocarbons (unburnt fuel) may be present in the exhaust.
• Carbon particles glow yellow in the combustion of a rich mixture and in severe cases
you may see black carbon smoke in the exhaust.
• When there is too much air in the mixture, it becomes more difficult to ignite, burns
slower, and is therefore less efficient.
• The diluted mixture burns with a pale flame.
• Misfiring may occur and hydrocarbon levels in the exhaust gas will rise.
• Carbon monoxide levels stay low because there is plenty of oxygen available to
convert it to carbon dioxide.
• On an engine with one carburettor/single point injection, a single Colortune plug will
enable the mixture to be observed in any cylinder for a quick check, or one cylinder at a
time for a more thorough evaluation of a multi-cylinder engine.
• Twin or multiple fuel systems can even be checked in this way (one cylinder at a time) but
it is time consuming and a comparison between cylinders at any instant is not possible.
• Multi-cylinder engines, particularly those with twin or multiple fuel systems are best
tested with a Colortune in each cylinder.
• Variations in mixture between the cylinders are easier to see and faults can be more
easily spotted and rectified.
• Nowhere is this better demonstrated than on high performance motorcycles - where
accurate calibration of at least eight settings on four individual carburettors is a highly
skilled task, done mainly by ear.
• A more accurate setting can be achieved with Colortune in half the time normally taken.
NOTE: A multi-cylinder engine with a carburettor (or injector) to each cylinder and a
single exhaust system can not be thoroughly tested with a gas analyser at the tailpipe.
The analyser only gives an average fuel mixture indication for all cylinders and no
warning of large mixture variations due to setting errors or indication of other faults.
• Modern engines with multipoint injection systems, and earlier high performance car
engines with twin or multiple carburettors, have a lot in common with the example
of the motorcycle given previously.
• Colortune gives a marvellous insight to the inner workings of all these engines, so
that errors and faults can be easily spotted. Despite its simplicity, Colortune is an
invaluable piece of equipment for the amateur or the experienced technician.
4.b. The Science
4.c. The practical details
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