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4.b.The Science
•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.
4.c.The practical details
• On an engine with one carburettor/single point injection, asingle Colortune plug will enable
the mixture to be observed in any cylinder for a quick check, or one cylinder at a time for a
morethorough 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.
•Amoreaccurate 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 equip-
ment for the amateur or the experienced technician.
