Simple Engine Tests With Colortune - Gunson COLORTUNE G4074 Mode D'emploi

6. Simple engine tests with Colortune.

Idle speed
With Colortune installed in a warm engine and the engine running at idle speed, a
regular flash of light, with a Bunsen blue flame, should be visible as the mixture is
ignited in the cylinder. On systems which have a slow running mixture adjustment, turn
the adjustment to explore the range of colour available. The position at which yellow
disappears leaving only blue, is the richest setting which should be used e.g. when
setting mixture on a simple motorcycle carburettor without an accelerator pump
device. Engines before 1985 should be set halfway between the point where yellow
flame disappears (say 4.5%CO) and the point where engine speed falls slightly (say
0.5%%CO). Engines produced since that date should idle at say 1%CO (almost at the
point where idle speed falls). These leaner settings progressed with engine
developments to give economy with low exhaust emissions.
Oxygen Sensor Mixture Control
On engines which have oxygen sensor mixture control, no yellow flame should be visible.
The flame colour should be blue with a slight variation as the oxygen sensor continually
trims the mixture every two seconds approximately (more rapid at higher rpm). A
yellow flame appearing in some cylinders is an indication of an air leak into the other
cylinder(s) or the exhaust manifold. The sensor tries to compensate for the excessive
oxygen level by increasing fuel input. (The same can happen if one injector is faulty but
symptoms may then be worse at higher engine speeds, whereas an air leak has less of
an effect when the throttle is opened.)
Part throttle
If the throttle is opened very slowly until the engine is running at perhaps half of the
maximum rated speed, the blue flame should become slightly lighter. The lighter blue is
due to a slightly leaner mixture (less fuel) arranged to give improved economy at part
throttle operation.
Note: At idle the same lean economical setting is difficult to achieve due to engine
inefficiencies at slow speed with the throttle virtually closed.
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