How the Carburettor Works
Most common carburettors (Weber, Keihin, Ford/Motorcraft, Dellorto,etc.) are 'fixed jet' designs. Several jets and 'emulsion tubes' are used to arrive at the correct amount of fuel for various running conditions. For example, there are small idle jets, strategically placed progression jets, and large, carefully calibrated main jets. More fuel is taken in for starting by lite 737t194h rally 'choking' the air intake - blocking it with a flap, either manually or automatically operated. For transient acceleration, when more fuel is required, a small diaphragm pump admits a carefully-controlled squirt.
Fuel injection systems, if you're smugly thinking to yourself that it's years since you've seen a carburettor, are not immune to problems - often the throttle body can be too restrictive for efficient breathing, and the airflow sensor must give accurate feedback on engine loading or the mixture adjustment can be as bad, or worse, than the equivalent carburettor system. As with fixed-jet carburettors, if the throttle body (venturi) is increased in size, engine breathing at high speeds is increased, however airflow velocity drops greatly at lower speeds and fuel droplets begin to drop out of suspension. (Fuel reaching the engine in a liquid state is no use; ideally the droplets are suspended in a dense fog.) To overcome the drawbacks of large venturis, many twin-barrel carburettors are set up with progressive throttles, so one barrel is inactive at low speeds. Many smaller engines are still highly efficient with a single carburettor. The transient throttle response of a FI system is often inferior because the injectors are unable to deliver a short burst of fuel, while remaining accurate for other engine load conditions.
SU carburettors are known as a 'variable jet' design, and are common on English cars. Other makes are Zenith/Stromburg and some Honda 'CV' designs. The principle used in all cases is a single large jet, varied in size by a tapered needle attached to a much larger piston. The piston moves in an aluminium 'suction chamber' - the top of which is easily recognisable - the inside is at atmospheric pressure, while under the piston is the 'venturi' - this varies in size according to the effective 'manifold vacuum' - when the engine is lightly loaded the venturi is small, the jet is almost closed and hence less fuel is used. On full throttle manifold vacuum drops away, the piston responds, allowing in more air and fuel as the jet simultaneously opens. The piston works against a calibrated spring, and the jet is regulated by a needle of calculated taper. For transient throttle response, the sudden movement of the piston would allow too much air in, causing hesitation. As with the accelerator pump of a fixed-jet carburettor, additional fuel is added, however this is smoothly achieved by damping the piston response with an oil-filled damper at its centre - this is topped-up through the familiar screw-on black cap.
For cold starting and running, additional fuel is admitted by moving the jet relative to the needle; on the HIF6 carburettor a bimetallic strip assists with this, cancelling the enrichment as the engine warms, there is also a cold-start jet in a small barrel-valve attached to the side of the main 'bridge'.
Adjusting the Carburettor
There are many books dedicated to adjusting SU carburettors - without going into too much detail, it is possible to set the mixture close to ideal in a few minutes. The HS6 carburettor has a large hexagonal nut - turn this up to weaken the mixture or down to enrich - while the later HIF6 has a recessed screw near the air intake. Turn this clockwise to enrich the mixture, back it out to weaken.
First ensure the ignition timing is set correctly - if this is over-advanced it will be much harder to judge a lean mixture. Then with the engine warmed up, lean out the mixture until the fastest idle speed is obtained. The idle speed will probably be varying by several hundred RPM. Enrich the mixture until a smooth idle is obtained, but not so much that the idle speed falls. If this process takes more than a few minutes, 'blip' the throttle to 'clear the intake'.
There is a piston lifting pin (for testing) on the right side of the carburettor, looking from the front of the car, near the flange of the suction chamber, protruding downwards. Lifting this with the finger while the engine is idling should cause the engine speed to momentarily fall, then pick up again as the piston is held. If the engine speed increases when the piston is raised, the mixture was too rich. If the engine dies, the mixture was too lean. In practice this test is not referred to in the Princess 2 manual; the ignition and lean setup of these cars tends to produce a 'too lean' result when the carburettor is correctly set. But you may wish to experiment anyway.
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