The carburettor plays a critical but straightforward role of mixing air and fuel to ensure the proper fuel ratio to generate power in an internal combustion engine.

While carburettors have been around as long as the internal combustion engine, many innovations were made, especially in the years between the wars.  

Some of the larger-engined vehicles of the late Fifties and  Sixties were quite complex, making them more of a challenge to rebuild for anyone less than a highly skilled mechanic.

Before beginning to overhaul any carburettor, time must be taken to read up on how the carburettor was designed, the  procedures that  need to be followed, and whether the rebuild will require specific tools.

If the engine of vehicle under consideraion is not firing correctly during assessment, after narrowing down the possibilities, most mechanics will eventually turn their attention to the carburettor.

Signs that the carburettor is faulty are reasonably straightforward.

  If the engine runs roughly and produces unhealthy looking black smoke, this generally means that the fuel/air mixture is too rich in fuel. Another indication of the same problem is a sooty mixture forming around the spark plugs.  

Most UK and European classic car engines were fired up with the aid of a choke.  When the vehicle is stationary, chokes should always be held in a closed position and only opened to close a valve known as the butterfly on the carburettor.

The butterfly valve limits the intake of air to the engine, thus creating a higher fuel-to-air ratio.

Problems with the choke can be caused by it being stuck in place, or it may have developed a "pinhole" causing it to lose buoyancy.

Another common problem with a carburettor occurs when the engine overheats, a fault that will cause significantly reduced acceleration.

Reduction in acceleration can be interpreted as a sign that the carburettor is providing an air-to-fuel mixture that is too lean.  A well-known side effect of this situation is when the engine produces a  sudden and loud popping sound from the throat of the carburettor. These mini-explosions can be a  cause for consternation for the driver, passengers and even passers-by.

Usually caused by air leaks caused by faulty gaskets in the carburettor, between the carburettor and the intake manifold, or even between the intake manifold and the engine.  A vacuum gauge should be used to test for air leaks.

Dirt, rust, or condensation in the float chamber or jets will cause an engine to sputter, run rough, and even stall. In this situation, complete disassembly and cleaning of the carburettor will be unavoidable.

Although restorers will usually be optimistic, the chances are that any classic's carburettor is probably worn, dirty and working less efficiently than it did in the engine's heyday.

Having it rebuilt will undoubtedly make a significant difference to the engine's performance in terms of smoothness, dependability and fuel economy.

Disassembling of the carburettor should be carried out carefully and all the parts clearly labelled.  The choke/butterfly valve assembly should also  be  clearly marked, indicating "which way is up" as it is possible to reassemble it in any configuration, although it will only work effectively if the parts are in their proper position

The next step is to remove the power jet, usually found in a large pipe near the accelerator pump.  After this stage is completed, the drain plugs, usually three in number, should be removed.

The plugs will jut out diagonally from the flow bowl.

The first stage of inspection should be to check for flatness, done by placing the air horn in position on the float chamber casting while ensuring the mating surface of the air horn isn't warped from over-tightening.  

If the air horn is found to be warped, it should be heated to a temperature of  350 degrees in an oven to be very flexible. The air horn should then be gently pressed flat in a vice, ensuring it is placed between two softwood blocks.

If this procedure cannot be carried out in-house, a visit to a machine shop will be called for, where the air horn can be dressed using a surface grinder.

The next stage should be to work the throttle back and forth and note whether the throttle body casting and shaft is worn. If the throttle body casting is worn, it will have to be reamed and bushed at a machine shop. If the shaft is worn, a replacement will have to be found.

The float should be thoroughly examined to ensure that there are no pinholes. These pinholes are impossible to repair and will need to be replaced.

Once all these steps have been taken, it is then possible to begin reassembly taking great care to reverse the order of the disassembly steps