PETROL ENGINES burn a mixture of air and tiny droplets of petrol, delivered in the form of a fine mist, and it is the carburettor which is responsible for creating this mist. It achieves this by drawing air through the carburettor throat over a small well of petrol (the jet). A constriction in the carburettor throat restricts the amount of air which can flow, creating a partial vacuum, which sucks petrol up into the air flow, creating the mixture, which is then drawn on into the combustion chambers.

The ratio of air to petrol must vary according to circumstances. As more air is drawn into the engine (as revolutions increase) then more petrol is needed to maintain the correct air/fuel ratio. During normal running, a ratio of fifteen to sixteen parts air to one of petrol (15:1 to 16:1) by weight is needed (air of course weighs very little).

During cold starts, a very rich mixture is needed of one part petrol to one part air (1:1) - roughly fifteen times as 'wet' as the normal running mixture.

During acceleration (such as it is with the Minor) a richer mixture is temporarily needed.

Different carburettors deliver differing strengths of air-fuel mixture in different ways. Some, called fixed jet carburettors, have more than one jet - one for normal running, another for cold starts and a little pump to supply extra fuel when the throttle is depressed sharply. The SU carburettor achieves all this with just one jet, and it's known as a variable jet or constant vacuum carburettor.

This is how it works.
To vary the amount of petrol available as engine revs increase, a tapered needle which fits inside the jet is raised, increasing the surface area of the pool of petrol. The needle is attached to a piston situated in the bell chamber atop the carburettor. As revs increase, the strength of the vacuum in the carburettor throat increases. this is fed to the bell chamber and causes the piston - and hence the needle - to rise.

In the SU carburettor, the constriction in the throat which creates the vacuum is caused by the bottom part of the piston. When the throttle pedal is pressed and the butterfly valve opens to allow more air into the combustion chambers, the strength of the vacuum is increased momentarily, but this pulls the piston upwards and so reduces the constriction in the throat and with it the strength of the vacuum - in this way, a constant vacuum is achieved. Of course, as the piston rises, more air can be drawn into the combustion chambers, so increasing engine revolutions.

For cold starts. the choke cable pulls the jet downwards, so that the surface area of the petrol 'pool' increases greatly from that of the hole in the jet to that of the hole in which the jet is situated - to give a rich mixture.

Maintenance
Easy - once a month, unscrew the black cap on top of the bell housing, and lift out the damper piston. Cheek the level of the oil in it's chamber and, if it's low, top it up to within 112" of the top with engine oil (use a pumping oilcan). Refit the piston. And that's it.

Carburettor Removal and Overhaul/Check
Disconnect the battery. Remove the air filter housing. Clamp the fuel line so that residual pressure doesn't spray petrol eve here and pull it from the inlet. Disconnect the accelerator and choke cables. Unbolt the carb.

The bell housing is secured bv three set screws. Remove these, remove the bell housing then pull the piston from the carb, taking care not to place any force on the needle (it bends easily). Clean the inside of the bell housing and the piston using petrol and a rag (observing safety precautions), and examine the inside of the bell housing for scoring (renew if there is any).

Cheek the needle for ridges by gently drawing your thumbnail along it - if you find a ridge, renew the needle and the jet. Visually cheek the jet - if it appears oval then renew the needle and jet.

Remove the float chamber top. cheek the needle and jet and renew if the needle has a wear ridge on it. Cheek the float for damage. If you have an older-type float then adjustment of the lever is possible - see a workshop manual for details. Clean out the sediment from the bottom of the float chamber.

Hold the throttle lever and feel for play in the throttle spindle; if this is loose, air can enter the carburettor throat, not only weakening the mixture but also affecting the strength of the vacuum. The carburettor body can be drilled out and have bushes fitted - work best left to an engineer. Alternatively, obtain an exchange reconditioned carburettor.

Common Problems
"I drove over a very bumpy surface, and ever since the engine runs roughly/stops". This is usually caused by the dirt which has settled out in the bottom of the float chamber being shaken up into suspension, and clogging up either the reservoir inlet jet or - occasionally - the main jet. In the short term, pinch the fuel delivery pipe and turn the engine over or - if it will run - run it, so that fuel is drawn from the reservoir and the level drops. After a few seconds release the pipe, so that fuel gushes into the reservoir. This usually unblocks the blockage.

In the long term, remove the reservoir cap (three set screws) and clean out all the rubbish (usually rust scale from a fuel tank which is on the way out) from the bottom of the fuel reservoir.

"My Minor uses petrol like I own an oil well". There could be two carburettor- related explanations to extremely high fuel consumption; if the choke is sticking on, your engine will be burning roughly fifteen times as much fuel as it should - cheek that the jet rises when the choke control is pushed fully home.

Perhaps more likely is a problem with the fuel reservoir (float chamber). If the inlet jet needle is wedged open (by dirt) or not sealing (due to wear - renew both the jet and needle), fuel will pump continuously - alternatively, the float could be damaged and not rising to cut off the fuel supply. Switch on the ignition; the fuel pump should issue a few audible clicks then stop - if it goes on pumping then open up the float chamber and see what's wrong.

"My engine simply won't run well". It is not uncommon for an engine to start and run normally at tick over, but to run erratically (or suffer pinking or other, problems) at any sort of speed. Cheek (or have checked) the ignition system before meddling with the carb. If the ignition proves OK, cheek for air induction, cheek the main needle and jet, cheek that the piston is able to rise and fall freely.

The piston and bell chamber can be cleaned using petrol and a rag - don't use abrasives for this, though, because you'd ruin the seal between the two. Pistons and bell housing are made for each other - renew them as a set.

The piston damper can fail if it's tiny bypass hole becomes blocked (renew it) and an old piston spring can become too weak to return the piston smartly - again, renew it.

Finally, if you renew the main needle and jet, the jet will have to be centred; a rather complicated-to-explain though not at all difficult process which any workshop manual will describe in full.