The carburettor can seem like a complicated device on the surface, treated by many contemporary mechanics as outdated and a bit of a lost art. Because of this, carburettors fall victim to a lot of myths such as its inefficiency, difficulty and unreliability.
Set up properly though, a carburettor can be almost every bit as good as electronic fuel injection. Carburettors are something I’m personally quite passionate about and I hope to provide a basic understanding on how they work in a simplistic sense.
The Word Carburettor comes from the word Buretta, meaning “A Measured Dose of fluid” and also the word for the glass syringes often used to measure Volume. A Car-Buretta is simply a device for a car that administers a measured dose of fluid, in this case fuel. In The Following examples I will be using the Weber DCOE Carburettor as an example due to its popularity and simplicity.
A carburettor contains various passages or “Circuits” to provide varying amounts of fuel depending on Throttle Position, Load, and RPM. It is important to remember that there is a difference between Vacuum and the Absence of pressure. Atmospheric Air Pressure is roughly 14.7Psi. Vacuum is negative pressure, where as the absence of pressure is anything between 0 psi and 14.7 psi.
One key feature of the carburettor is the “Venturi”: a tapered Bore which acts as a bottle neck. The throttle butterfly connected to the accelerator Pedal is located within this tapered bore. As the Piston within the engine travels downwards and the exhaust valve opens its creates an area of lower pressure than atmospheric inside the intake runner. Atmospheric pressure pushes the air down the Venturi and into the cylinder.
As the air is forced into a smaller opening, it speeds up. This is referred to as the Venturi effect. The Weber carburettor contains 2. A main Venturi and Auxiliary Venturi . The Auxiliary Venturi is a smaller one that sits within the main Venturi. This is to aid atomisation and distribution of the air/fuel mixture when air speed is very low.
Within the carburettor is an “Idle Jet” this receives fuel from within a small drilling. A Jet is the term given to the Brass nozzles within a carburettor that govern the amount of fuel delivered. The end of this jet is exposed to the air travelling through the Venturi. The air moving quickly over the end of the jet creates a negative pressure area and the fuel within the carburettor “float Bowl” is drawn through the jet which atomises the fuel, Mixing it with the air. The sizes of these jets can be changed to allow more or less fuel. The Idle Jet (along with progression ports) are responsible for the air/fuel mixture up to about 2000rpm.
You can actually test this at home. By filling a glass with water and placing a straw in it. By blowing across the top of the straw at the right angle you can actually watch the water get drawn up the straw and atomised. Below you can see the Auxiliary Venturi within the Main Venturi. The main jet’s supply can be seen protruding within this.
The Float Bowl is simply a chamber that retains fuel for use by the carburettor. It receives Fuel supplied via the fuel pump. Within it is a Float that open and closes a Needle and seat valve so to maintain correct fuel level within the bowl. Float height and effectively fuel level can be altered and is a key part in correctly tuning a Carburettor.
After this point air speed drops and the Idle Circuit ceases to work. The Emulsion Tube is the next part of a basic carburettor. Located central of the DCOE weber carburettor It has an Air Correction Jet at one end and the “Main Jet” at the other. Atmospheric pressure forces the Fuel into the Emulsion tube and atmospheric air pressure supplies air to the Air Jet at the other end. Both are Fed through holes drilled in the side of the emulsion tube and supplied to the Choke Venturi. This controls the air/fuel mixture for higher engine speeds. There is a series of wholes on the side of the emulsion tube, at idle, the holes are covered by the fuel level and no air can pass through them, as engine speed increases the fuel level drops and the holes are progressively uncovered, allowing air to fix with the fuel and pass though.
In addition to these Circuits the Carburettor has an Accelerator Pump. Literally a Diaphragm pump connected to the throttle butterfly via an internal linkage. This is to provide a momentary additional fuel supply when the accelerator pedal is quickly depressed. It receives Fuel from within the Float Bowl via the Accelerator pump jet.
A common Misconception with carburettors is they require frequent retuning. This is a common misconception. Mixture screws are held in place by springs. If your carburettor is mounted Via Rubber gaskets and these springs are in good condition your car should not require adjustment very often. If you have purchased second hand carburettors it is always worth replacing the gaskets and cleaning it out as well as replacing these springs to ensure trouble free motoring. I have solid mounted weber carburettors before on weekend cars and race cars, and while It performs quite well, Though the vibrations caused can lend themselves to the car slipping out of tune.
There is a lot more to the Carburettor, and tuning should always be left to an experienced Mechanic. Hopefully This article has served to provide a basic understanding of the carburettors workings.
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