Gunson COLORTUNE G4074 Mode D'emploi page 30

Full throttle
When maximum power is demanded (at full throttle position) it is normal to again have
a slightly richer mixture, at most engine speeds there will be a yellow flame. Engines
with modern electronic engine management systems which have more precise control
would not usually give a yellow flame at full throttle, except at times of rapid
acceleration.
Rapid acceleration
When the engine is running slowly at idle and there is a sudden demand for power
(throttle rapidly opened) this can cause an unstable condition and the engine may stall.
A rich and easily ignitable mixture helps to avoid this, so it is common to see a yellow
combustion flame with rapid engine acceleration. A special fitment may for example be
provided on a carburettor to achieve this extra fuel delivery - this is commonly called
an accelerator pump.
Engines with modern electronic engine management systems which have more precise
control would give a very brief yellow flame which quickly reverts to blue. Less
sophisticated systems may give a yellow flame throughout the period of acceleration.
Very simple carburettors found on small motorcycles and garden machinery may have
no acceleration device on the carburettor and will need a richer mixture setting at idle
to avoid engine stalling or a "flat spot" during rapid acceleration.
Cold starting
A rich and easily ignitable mixture is also provided for starting a cold engine. If a
manual cold start device (choke) is fitted, a yellow flame should be visible in Colortune
when this is operated. This applies with a hot or cold engine. If enrichment is by
electronic control unit (ECU) or auto-choke, test for a yellow flame when the engine is
cool / cold. Try to keep the test brief to avoid carbon deposits on the Colortune glass.
Some exceptions to all the above conditions may be found, for example small engines
used in lawn mowers or chain saws may have simple fuel systems which are less able to
provide the ideal fuel / air mixture. Electric generators with speed governors do not
experience rapid acceleration so should have a blue flame under all operating
conditions.
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