How it Works - Carburretors

When I started to write this article I had intended it to be just one article but the more I looked into it the more I realised I needed to set out a few design points first.  I hope I don’t lose you with the theory part, it will be useful to understand why the car does not perform when you have a blocked jet, or your petrol level is too high in the float chamber or you can not get the tickover to run smoothly.

The carburettor has three main functions; to deliver a mixture of petrol and air  in such a form that it can be burnt readily in the exceedingly short time available; to ensure that the ingredients are in the correct proportions; and to provide the driver with a method of controlling the amount of mixture going into the engine.. 

It should be said that a liquid can only burn on its surface (where it is in contact with oxygen) and as the time available at even moderate speed is in the order of 1/100 the of a second there is a need to present the liquid in such a form that it has maximum surface area.  In other words a very large number of very fine droplets.  What should not occur is for the fuel to turn into a vapour  as this takes up far more space than droplets and therefore reduces the amount of air (oxygen) in the volume of mixture. This would lead to less power.

Mixture Proportions
Besides supplying the fuel as a finely divided spray suspended in air, the carburettor must ensure that the proportion of fuel to air is correct.  Given the chemical composition of the fuel one can arrive at a certain number of pounds of air as being needed to burn one pound of the particular fuel.  Unfortunately it is not quite so simple as that.  About 14:1 ratio is chemically a correct mixture with most ordinary petrol available in the 1930’s/40’s.  For benzole the corresponding figure is about 13:1 alcohol about 8:1.  Modern fuels are substantially different to the fuels available at the time our cars were made. 

Practical tests showed that the greatest power is developed when the proportion of fuel to air is rather greater than the theoretically correct value.  However, tests also showed that the greatest economy occurs when the mixture is a good deal weaker than the theoretical value.  From this it might seem that we must choose which particular virtue we want and adjust the carburettor accordingly.  However, that would not satisfy the many of us who, want the maximum power and the maximum economy of fuel with one and the same setting.  

There are ways of providing this based on the simple fact that when a driver wants full power he opens the throttle as wide as it will go.

The carburettor is arranged  to provide the rich mixture for maximum power only when the throttle is wide open and to arrange a weak mixture for economy at other times.  Added to these requirements is the fact that when a cold engine is being started, and for idling, it needs a mixture much richer than that for normal running.  Consequently there must be some method of making it richer for starting (choke) and some arrangement must be made for the mixture to be a good deal richer when the throttle is practically closed (slow running mixture jet).

There is also some need for an extra but temporary flow of fuel as the throttle is opened with the engine already running. This is because with the engine running and the throttle partly closed (at 20 or 30 mph on a level road) the pressure inside the induction pipe is less than that of the atmosphere outside; and so the petrol evaporates more readily and very little of it hangs about the induction pipe in liquid form.  When the throttle is opened, however, the induction pressure rises nearer to atmospheric pressure and some of the fuel mixture sticks to the manifold walls weakening the mixture briefly unless there is some device to add a corresponding amount of fuel to the mixture at the instant when the throttle is opened.

Even though they are required to meet several requirements carburettors are relatively simple from our point of view but not necessarily from a designer’s point of view.  I realise that the term suction is incorrect and it should be the atmospheric pressure moving into an area of lower pressure but we all talk about the engine sucking in the air so I will stick with it.

carbs 2

The Simplest Carburettor
If we attach a short pipe to the inlet port of an engine, air will travel from the atmosphere into the engine when the piston moves down the cylinder on the induction stroke.  By reason of the speed of the air in that pipe the pressure there will be below that of the atmosphere; therefore, the difference between this pressure and the atmospheric pressure will cause petrol to flow into the pipe if we arrange for a small hole (a jet) in the side of the pipe, the hole being connected to a reservoir of petrol which is exposed to atmospheric pressure.  In other words the lower pressure in the carburettor tube sucks petrol into the tube. 

By arranging this reservoir so that the level of the petrol is just at (or very slightly below) the level of the jet in the side of the air pipe, we shall be sure that no petrol will flow into the engine, except what is due directly to the pressure difference caused by the flow of air. 

Unfortunately we can not just leave it at this as the difference in weight between petrol and air means that the mixture ration would change depending on the speed of the flow through the carburettor, the mixture gets progressively richer as the air-speed rises.  Although we need this richer mixture as we open the throttle it does not meet the needs of the engine.  We need some form of mixture control. 

Mixture Control
Potentially this control could be done by the driver but it would not be accurate enough to meet the engines needs.  No control would be correct some of the time whereas a manual control could be wrong all of the time!

Some carburettors use a mechanical method of correcting mixture to meet the needs of an engine, an SU carburettor has a piston effected by the ‘suction’ of the engine which adjusts the mixture in relation to the engines demands.  The majority of Austin Sevens use a Zenith carburettor which has a form of automatic control so this is the one I shall attempt to describe.

The Zenith uses a small additional jet which ‘leaks’ additional petrol into the mixture directly into the air pipe leading to the engine. The mixture strength due to this second jet will vary quite a lot as the air-speed changes, and that it will become progressively weaker as the air-speed increases. This result  is just the opposite of what happens in the simple jet-in-tube carburettor. Therefore, a combination the two jets results in that while one part provides a' progressively richer mixture, the other gives a weaker one.  

By suitably controlling the two jets (by jet size) the overall result should be a mixture strength which remains constant at all air speeds.  The Zenith 26VA carburettor uses this principle  but both jets are submerged but produce the same end result.  When you look into the bottom of the float  bowl chamber you will see two jets; the main jet is the smaller one and the other jet is the compensation jet.

carbs 4


A second article deals more specifically about the Zenith 26VA carburettor in part 2 which gives information on how the two main jets work.


This article, written by Malcolm Watts, originally appeared in Seven Focus Dec 2006 pp16-18.


See also:

Fuel System Explained

A.C. Mechanical Petrol Pump 

Carburettor Jet

Carburettor Throttle Spindles (Zenith 26VA)

Carburettors Explained