Page 209 - 05. Subyek Teknik Mesin - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

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Engine systems 193

Figure 2.228 Throttle butterfl y

Figure 2.229 Fuel level is control by a fl oat and needle valve

manifold. A valve to meter the air fl ow is fi tted at the base of the carburettor just
in front of the inlet manifold. This valve is called the throttle and it consists of a
round plate on a spindle ( Fig. 2.228 ). The spindle has a lever attached to one
end and this is connected directly to the throttle pedal with a cable or rods. The
throttle restricts the air fl ow in all positions except when wide open, and this
gives a range of variable pressures in the carburettor and the inlet manifold.
The basic carburettor consists of the venturi, through which the air fl ows,
and the fl oat chamber, which holds a supply of petrol at a constant level in
relation to the supply beak in the venturi ( Fig. 2.229 ). The level of petrol in the
fl oat chamber is maintained by a needle valve that is lifted onto its seat by the
fl oat so that it stops the fl ow when the chamber is full. As petrol is used the
level drops, the needle valve opens and the fl ow of petrol into the chamber
resumes. In this way, a constant petrol level is maintained. The fl oat level should
be checked and adjusted if necessary, if problems occur or if the carburettor is
stripped for cleaning.

The main jet in the fuel feed to the venturi forms a restriction in the petrol fl ow
and by virtue of its size acts as a metering device ( Fig. 2.230 ). The venturi is a
tube with an inward curving restriction. Air fl ow through the venturi speeds up as
it passes through the restriction. The effect of this is to reduce the air pressure at