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The SU Starting Carburettor

In "The Automobile Engineer" October 1937 the article entitled "The S.S. 2-Litre 'Jaguar'" states that the S.S. company pioneered the use of the SU automatic auxiliary starting carburettor. We may at times wish that we could disable this little device, and some have fitted switches for this purpose, but there is little doubt that starting the S.S. in 1937 was a rather simpler procedure than that described for starting the 6 cylinder Bentley of ten years previous. Take look at the Bentley handbook from 1927.

For my own car I tend to favour keeping the starting carb as lean as possible. If I have used the car the day before then she will normally start first go but if several days have elapsed then she normally doesn't fire until the third try. I don't like adding switches to an original layout but the panel lamp switch on the pre-war cars operates independently of the lighting switch so I've arranged for it to supply both panel lamps and starting carb. Once the thermal switch kicks out then clearly the panel lamp switch just performs its normal function.


Before anyone points it out, I know that in the video I've got the wrong cylinder head (from a Mark IV) for my SS and the wrong water manifold but this has now been rectified. The SS manifold is longer and has the thermal switch on the under side near the front.

The starting carburettor is a separate unit which draws its fuel from the float chamber of the rear main carburettor. It is activated by the thermal switch mounted in a slot on the water manifold. This is under the front end (as in the engine photo above) in earlier cars and on the side between the two main carburettors in later models.

When the device is operated, air is drawn from the atmosphere through the air intake P and into a chamber at Q and is mixed with fuel passing through the jet C. The mixture then passes upwards past the shank of the needle, through a passage, and so past the aperture provided between the valve H and its seating. From here it passes directly to the main induction manifold within the cylinder head.

When the solenoid J is energised the iron core I is raised carrying with it the ball-jointed disc valve against the load of the conical spring thereby opening the aperture between valve H and its seating. Any leakage between this valve and its seating would allow the device to operate when not required and affect the idling setting of the main carburettors. If the solenoid is energised while the engine is idling the valve will not normally lift owing to the high manifold depression; the act of opening the throttle will reduce manifold depression and allow the device to operate.

The fuel level in the starting carburettor is controlled by the rear main carburettor float chamber A. It can be seen from the illustration that this results in a reservoir of fuel remaining in the well of the starting carburettor. When starting from cold this fuel is drawn into the induction manifold to provide the necessary rich mixture.

When the valve H has lifted, the needle disc chamber is in direct communication with the inlet manifold and the depression, dependent on throttle opening, varies the position of the needle D by exerting a downward force upon the suction disc N and needle assembly.


(a) At idling the relatively high depression will draw the needle into the jet until
       the needle head G abuts against the adjustable stop F.

(b) At larger throttle openings a reduced depression is communicated to the
       needle disc chamber and the spring will tend to overcome the downward movement
       of the needle thus increasing mixture strength.

Tuning of the starting carburettor is confined to adjustment of the stop nut F which limits the downward movement of the needle and is carried out with the engine running at normal temperature and with the main carburettors already correctly tuned.

Proceed as follows:

1. Because the engine has reached normal running temperature the thermostat
     will not be energising the solenoid so you will need to short the thermostat
     connection with a separate wire to the thermostat mounting screws or some other
     convenient ground connection.

2. Open the throttle momentarily to allow the valve H to lift.

3. Adjust the stop nut F with reference to the graph as follows:
    (a) Turn nut F clockwise (to weaken) until the engine begins to run erratically.
    (b) Then anti-clockwise (to enrich) through the phase where the engine
           speed has risen markedly to the point where over richness results in
           the engine speed dropping to between 800 and 1,000 rpm with the
           exhaust gases noticeably black in colour.