Technical Pages - FUEL SYSTEM
The SU Carburettor

The S U Carburettor is a masterpiece of simple yet effective design- this is how it works.
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 15 to 16 parts air to 1 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 1 part petrol to 1 part air (1:1) - roughly 15 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. Whenthe 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.


SU Carburettor Tuning

To adjust and tune the SU carburettor proceed as follows:

(1) Start and run the engine until it reaches normal operating temperature.
(2) Remove the air cleaner.
(3) Top up the carburettor dashpot with SAE 20 grade oil.
(4) Unscrew the throttle adjusting screw until the face of the screw clears the stop.
(5) Check the carburettor spindle lever position so that when engaged with the countershaft the butterfly is in the closed position. Should adjustment be necessary, slacken the lever clamp bolt, reset the lever and tighten the clamp bolt.
(6) Screw the throttle adjusting screw down one and a half turns only.
(7) Screw up the jet adjusting nut until the jet is flushed with the bridge of the carburettor. DO NOT OVERTIGHTEN.
(8) Next screw back the jet adjusting nut two complete turns only
(9) Again run the engine for a few minutes so that it is at normal running temperature and adjust the throttle adjusting screw until the engine idles at a speed of approximately 550 -600r.p.m. Slowly turn the jet adjusting nut until the fastest idling speed obtained with no signs of engine misfiring. Screwing the nut up weakens the mixture and screwing it down enriches the mixture. It will be observed that as the mixture is adjusted, the engine will most likely run faster and therefore the speed will have to be adjusted back to the desired idling speed.
(10) Check the mixture strength by lifting the carburettor piston 1/32 inch using the little lift pin located on the side of the carburettor and check for one of the following conditions:
If the engine speed increatooses and continues to increase, it is an indication that the mixture is too rich.
If the engine speed decreases immediately it is an indication that he mixture is too weak.
If the engine speed momentarily increases very slightly the mixture is correct

If the carburettor is not fitted with a lift pin unscrew the top of the piston damper and pull the damper out quickly. The drag of the oil will lift the piston.
(11) When the mixture is correct the exhaust notes should be regular and even, if it gives a splashy sound with a colourless exhaust it is an indication that it is too weak. Alternatively, if there is a regular and rhythmical type of misfire accompanied by a dark exhaust it is an indication that the mixture is too rich.