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Technical: 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.

In 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!

While there are many different types of carburettors: single-barrel; double-barrel; four-barrel; side-draughts and SU, the function of all is essentially the same - to cause the fuel to vaporise. As always, the perfect fuel mixture is 14.7:1 - although a carburettor can operate anywhere between 13 and 17:1 purely because it is a mechanical system. Jetting is done in the hopes of finding the perfect mixture.

The different stages of the carburettor are: idle, part-load and full-load. At idle, the butterfly / throttle valve is almost closed. High vacuum takes place after the butterfly, drawing in fuel at the mixture screw where the CO’s are adjusted. As soon as acceleration takes place, the vacuum at the venturi takes over and starts drawing the fuel into the carburettor.

The more acceleration, the higher the air flow rate through the venturi and the greater the suction force drawing fuel into the venturi. Every time the driver accelerates, the timing at the spark advances because of the vacuum in the intake manifold. With advance spark, you get more air and more fuel for acceleration and to prevent flat spots. This fuel is injected through a speed pump in the carburettor, which helps to eliminate flat spots when pulling away. Airflow is also controlled through a primary and secondary venturi, giving the driver more power at top-end.

The fuel feed to the carburettor comes from a fuel pump bolted to the engine, which ensures constant fuel flow to the carburettor. The carburettor needs to prevent excess fuel from entering it, which would otherwise cause flooding or over-fuelling. This is performed by a float chamber with a float 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. 

We all know that when a vehicle is cold started in the morning, it will run unevenly or have a hard start until it gets to the correct working temperature. In cars with carburettors, this problem is dealt with by a choke, which is situated above the carburettor. The choke allows the driver to limit the air-flow through the carburettor, thereby enriching the fuel mixture for better idling while the vehicle is cold. As soon as the vehicle has reached its normal operating temperature, the driver can open the choke and the carburettor will return to normal working procedures. There are two types of chokes: mechanical and electrical.

Next month we’ll talk about the motorised throttle body and accelerate by wire technology.