Gunson COLORTUNE G4074 Mode D'emploi page 32

which gives this carburettor its name. The slide is attached to a needle running in a jet
in a similar way to the variable choke carburettor but this does not give a constant
depression as the slide position is governed by the throttle position selected at the
time, not by a diaphragm.
Mixture is governed by the shaped bottom of the slide at lower speeds/light loads, the
needle taper and position at mid speeds and the main jet size at all engine speeds when
on full throttle.
Carburettor fuel systems fitted to car engines
Fixed choke carburettors.
These are broadly similar to the basic carburettor fuel system described except no
adjustment is normally provided for the fuel fed by the main jet system.
More sophisticated idle and main circuits are used, with additional fuel feed circuits.
Also there are air jets and emulsion tubes which provide a graduated addition of air
bubbles (emulsion) to give greater control of fuel flow characteristics.
Idle speed adjustment is normally by a throttle stop screw, but sometimes this is locked
and an adjustable throttle by-pass passage is used.
This type may have two fixed choke carburettors combined in one casting to provide
either -
A twin choke carburettor where both throttles open simultaneously -generally for
improved power output (two twin choke carburettors may be fitted to a high
performance four cylinder engine).
A twin choke (progressive) carburettor where one choke is used for most low speed
driving and the other opens in addition for high speeds. This gives improved economy
and smooth engine response
Variable choke carburettors.
These have some similarity to motorcycle variable choke carburettors but a separate
idle circuit is not usually provided. Two basic types are used with control of the air
valve being either diaphragm (Stromberg/CD) or piston in cylinder type (SU).
A constant depression (low pressure) is maintained on a tapered needle and jet system
which is attached to the air valve/carburettor piston. This air valve rises as the throttle
is opened and then at full throttle it will continue to rise further as the engine speed
also increases. This gives accurate mixture control over the whole range of operation.
Enrichment for acceleration conditions is provided by controlling the rate, at which the
air valve rises, with an oil damper, (the oil may require topping up regularly).
The fuel needle is machined with a very accurate taper, individually selected for each
engine design. Spring loaded types of needle are more accurate when new but they rub
against the jet giving errors when worn (e.g. over 50,000 miles 80,000 Km). Appearance
of the combustion flame will be rich at the operating conditions where the needle is
worn (usually idle / part
throttle). If the mixture has been adjusted to normal at idle, weak mixtures will then
occur under acceleration.
Fuel injection systems
Drawing it in with air under low pressure). The amount of fuel should be in proportion
generally with the amount of air going into the engine, but this is adjusted slightly for
economy at part throttle, higher power at full throttle and other less significant
reasons. A three dimensional map of the requirements, for a specific engine, is drawn
and it is then the job of the injection control system to satisfy this requirement.
Determination of the quantity of air going into the engine is itself a difficult task under
conditions of varying temperature, varying pressure, and with extreme disruption to
the flow caused by valves opening and closing at an incredible rate. Some systems
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