Carlo du Plessis
carlo’s technical page
Carlo is the director of
Bosch Cencar and the
chairperson of the Bosch
Service Marketing Forum.
Carburettors and
Throttle
Valves
This month we discuss the workings of the throttle
valve, which gives the driver control over engine
speed in vehicles equipped with carburettors.
➲ A basic diagram showing the internal workings of a carburettor
I
n older cars, a carburettor
is used to create the
required fuel mixture,
which takes place in the
intake. Whether the car is
hot or cold or at idle, low
revs or high revs, the mixture
must be 100 per cent right to
avoid jerking, losing power or
increasing fuel consumption.
The latter symptom translates
into unburnt fuel leaving the
exhaust, which is not good
for the environment – or the
pocket!
and needle (needle and seat)
located inside the carburettor.
When the carburettor’s float
chamber is filled to capacity, the
float pushes the needle into its
seat, blocking off fuel supply.
The fuel mixture at part-load and
full-load is determined by the
jetting of the carburettor. There
are air-correction jets and main
jets fulfilling this role. Each and
every carburettor has different
jet thicknesses, according to the
specifications of the vehicle in
question.
➲ A downdraught, single-bore carburettor fitted to a Nissan 1400 bakkie
While there are many different
We all know that when a vehicle
types of carburettors: singleis cold started in the morning, it
the suc tion force drawing fuel into the
barrel; double-barrel; fourwill run unevenly or have a hard start until it
venturi. Every time the driver accelerates,
barrel; side-draughts and SU, the function
gets to the correct working temperature. In
the timing at the spark advances because
of all is essentially the same - to cause the
cars with carburettors, this problem is dealt
of the vacuum in the intake manifold. With
fuel to vaporise. As always, the perfect fuel
with by a choke, which is situated above
advance spark, you get more air and more
mixture is 14.7:1 - although a carburettor
the carburettor. The choke allows the driver
fuel for acceleration and to prevent flat
can operate anywhere between 13 and 17:1
to limit the air-flow through the carburettor,
spots.
purely because it is a mechanical system.
thereby enriching the fuel mixture for better
Jetting is done in the hopes of finding the
This fuel is injected through a speed pump
idling while the vehicle is cold. As soon as
perfect mixture.
in the carburettor, which helps to eliminate
the vehicle has reached its normal operating
flat spots when pulling away. Airflow is also
The different stages of the carburettor are:
temperature, the driver can open the choke
controlled through a primary and secondary
idle, part-load and full-load. At idle, the
and the carburettor will return to normal
venturi, giving the driver more power at
butterfly / throttle valve is almost closed.
working procedures. There are two types of
top-end.
High vacuum takes place after the butterfly,
chokes, mechanical and electrical.
drawing in fuel at the mixture screw
The fuel feed to the carburettor comes from
where the CO’s are adjusted. As soon as
a fuel pump bolted to the engine, which
Next month we’ll talk about
acceleration takes place, the vacuum at the
ensures constant fuel flow to the carburettor.
venturi takes over and starts drawing the
the motorised throttle body
The carburettor needs to prevent excess
fuel into the carburettor.
and accelerate by wire
fuel from entering it, which would otherwise
cause flooding or over-fuelling. This is
The more acceleration, the higher the air
technology.
performed by a float chamber with a float
flow rate through the venturi and the greater
| words in action
44
april 2014