Carburettors without a separate idle mixture circuit are typified by the SU and CD
horizontal variable venturi types. Mixture throughout the speed range is governed by a
long tapered needle in a jet. Lowering the jet or raising the needle by manual adjustment
causes a richening of the mixture throughout the operating range of the engine.
Figure
7
SU
H.I.F.
TYPE
JET
The following notes describe particular types of carburettors and the methods of
adjustment.
SINGLE FIXED VENTURI TYPE
This is one ofthesimpler forms of carburettor with asingle air inlet and throttle plate,
with a variety of air and petrol metering jets and channels. The main jet and associated
main air jet and emulsion tube etc, provide an aerated "emulsior?" which is fed to the
venturi at speeds above idle. This already aerated fuel and air mixture breaks down
further in the airstream.
The idle circuit is separate and also has a fuel and an air jet which feed an aerated
mix to a drilling downstream of the throttle plate, further drillings are found in the area of
the throttle plate. Just above the idle drilling would be found "progression" holes which
are progressively uncovered by the movement of the throttle plate and increase the fuel
flow when exposed to the manifold depression (or vacuum). This supplements idle fuel
flow until the main fuel discharge in the venturi is well established. All of these fluid
circuits are fed from a small reservoir of fuel whose level is controlled by a float and
needle valve.
SINGLE VARIABLE VENTURE TYPE
This type of carburettor consists of a single air inlet (but more than one carburettor is
sometimes fitted: see Section
8,
Twin and Multiple carburettors), a throttle plate (or
butterfly), and an air valve or a piston which closes off the air inlet to which is attached a
tapered fuel metering needle. This needle runs inside a fuel jet which draws mixture from
a small reservoir of fuel. The level of fuel is controlled by a float and valve.
At idle, when the throttle is nearly closed, the air valve is almost completely closed
and the tapered needle which is attached to it restricts the flow of fuel to a great extent.
As the throttle is opened the air valve is drawn upwards allowing more air
toenter and the
needle is drawn out of the jet allowing more fuel to flow. If the throttle is opened fully at
low RPM the air valve rises about halfway. As the engine speeds up and draws in even
more air the air valve will continue to rise.'
'Thus the top half of the needle governs part
throttle mixture and the lower half (slim end) governs full throttle mixture.
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