Setting Idle Speed and Mixture

The idling speed itself is governed by the setting of the throttle-stop screw, while the idling mixture is controlled by the jet-adjusting screw at the base of the carburetter float-chamber, and when the idling speed is being set or adjusted these two controls must be used in conjuntion with each other.  Engine speed is set in the idle postion by the throttle-stop screw, while the jet-adjuster determines the richness or othewise of the fuel/air mixture.

To set the idling speed, the air-cleaner and damper must be removed, and the air-valve must be held down on the bridge in the throttle bore.  Using a coin, which will be easier than a small screwdriver, the jet-adjuster should be screwed up until the jet is felt to contact the underside of the air valve, at which point it should then be turned down again three full turns.  By doing this, you can establish a "working position" for the jet.

With the engine at normal running temperature, the idling speed should be set by using the throttle-stop screw to a speed of 600-650 r.p.m. and, by careful and gradual adjustment of the jet-adjuster, vary the mixture until the engine runs with a  smooth, regular beat.

To check the final setting, lift the air valve a very small amount - no more than 1/32 in.-and if the engine speed rises appreciably, the mixture is too rich; if it stops, the mixture is too weak.  If the setting is right, the engine speed will either remain unaltered or may drop very slightly.  Turning the jet-adjusting screw clockwise will weaken the mixture; turning it anti-clockwise will make it richer.

You should bear in mind that idle "quality" depends not only on the correct carburetter setting, but also on the general condition of the engine and ignition timing, spark-plugs and tappet adjustment should also be checked; any leaks at manifold joints will also affect the carburation while very old carburetters may have to be replaced due to internal wear, or fitted with a new throttle spindle.

Jet Centralisation

The carburetter cannot function effectively unless the jet orifice is placed so that the metering needle can move freely and centrally within it, and whenever the jet assembly is removed, for whatever reason, it must be re-centred.  A quick check is to lift the air valve by means of the spring-loaded pin found at the side of the instrument, opposite the throttle butterfly and beneath the diaphragm housing.  If the valve will fall back freely, the jet is central; if not, it must be reset.

This is done by lifting the air-valve and fully tightening the jet assembly, then screwing up the orifice adjuster until the top of the orifice is just above the bridge in the throttle bore.  Then, to release the orifice bush, slacken off the whole jet assembly by about half-a-turn and allow the air valve to drop back: the needle will then enter the orifice and in doing so will automatically centralise it.  It may be necessary to assist the air valve to drop with more freedom by unscrewing the damper and sticking a thin rod in the dashpot.

The assembly can now be retightened, checking frequently as you do so that the needle remains in the orifice.  This can be checked by lifting the air valve about 1/4 in. and letting it drop back freely, when the piston should stop firmly on the bridge.  The idling speed/mixture will now have to be reset, of course.

Synchronsing Twin Carburetters

Twin-carburetter installations must be carefully synchronised for best results and while procedure is quite straightforward the fact of correct synchronisation is more complex on the CD instrument because of the variable-jet feature.  The first move is to examine the action of the two throttles, making sure that both throttles close completely when the throttle-stop screws are slacked right off.  Loosen the clamp-bolts on the throttle-spindle couplings and then, with each throttle completely shut, make sure that the fast-idle screw is clear of the cam on the side of the carburetter body.

The throttle-stop screws must now be screwed in, first to the point at which the ends of the screws are just contacting the casting and then by a further 1 1/2 turns each.  This will open the throttles by an equal amount and provide a basis for setting idle-speed.

Regulate the jet-adjusting screws three turns down from the point at which the jet orifice comes into contact with the base of the air valve, treating each carburetter as outlined in Section 2 under "Setting Idle Speed and Mixture", and check that both cold-start levers are fully off against the stops with the dashboard control pushed fully in.  If necessary, adjust the couping and control wire until this is so.

Next start the engine and run to normal working temperature and then set the idling speed as outlined in Section 2; both throttle-stop screws must, of course, be set in unison.  When applying the piston-life test-by lifting the air-valve 1/32 in. by means of a cycle-spoke or some similar tool-the rise or drop in engine speed will apply only to the two cylinders served by that carburetter, and the object will be to achieve an identical effect on each pair of cylinders.  (Some carburetters have integral lifting pins).

As a final check, you should make sure that the hiss from each carburetter intake is equal in intensity.  A rubber tube, one end placed in the intake and the other end held to the ear, will make it easier to detect any difference, while a number of suitable tools is on the market, such as the German PSW tool marketed in the United Kingdom by Motor Books and Accessories.