problem. Wherever possible, the user is advised to consult a detailed workshop manual
for the particular car, but the following notes are provided for use when such information
is not available.
Firstly it should be mentioned that it is a legal requirement that all carburettors have
either a method of sealing the adjusting screws, or require the use of a special tool to
enable adjustments to be made. In some ways this has been a retrograde step, as
although it may stop "tinkering" by an unskilled owner, wear of the various parts takes
place during the life of the car, and mixture adjustment is frequently ignored until it
becomes troublesome in terms of starting, performance or economy. These seals are
usually thin metal or plastic plugs which are destroyed on removal, and are usually
removed using a sharp screwdriver bit or short self-tapping screw. Other types offer a
limited adjustment which can be increased by removal of a cap, and some cover seals
have a removable centre section allowing access to the adjustment. The vehicle
manufacturer will invariably recommend that the seals are renewed after adjustment, but
this is frequently ignored by the service trade or vehicle owner, particularly after the
service warranty has expired. In some countries, particularly the
USA
and Japan, this
would be an offence.
Carburettor types can be divided into those that have a separate fuel circuit for idle,
and those without a separate idle circuit. Figure
3
shows a typical arrangement of
separate idle circuit. The mixture is made richer
(i.e. CO higher by screwing the screw
out)
IGNITION ADVANCE
,.
DRILLING
Figure
3
IDLE MIXTURE
PROGRESSION IDLE MIXTURE
(CO) SCREW
HOLE DISCHARGE
In such carburettors, idle speed can be of the adjustable throttle stop type, an
example being shown in Figure
4.
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