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Adjusting a Stromberg carburettor

The Stromberg CD carburettor , like the SU (See Adjusting an SU carburettor ), may be a constant-depression carburettor, hence the CD. The two work in much the same way, but the Stromberg differs in having an air valve – commonly called the piston – surrounded by a rubber diaphragm within the dashpot. The size and sort of a Stromberg are given by numbers and letters. Numbers like 125, 150 and 175 are sizes: they mean a choke diameter of 1 1/4, 1 1/2 and 1 3/4 in. the size makes no difference to the way you tune the carburettor.

Types include CD, CDS, CD2S and CD3, all used on older cars made before anti-pollution laws demanded emission-control fittings. Later types with emission control have the letter E, as in CDSE and CD2SE. Tuning methods vary according to type, but Strombergs are not stamped with their type marks. Information within the car handbook should assist you to spot your type.

Air cleaners

Before starting to tune the carburettor, check out all the other systems like sparkplugs, contact-breaker gap and therefore the ignition timing (See the way to drain engine oil and take away filter ). Although valve clearances seldom need resetting between major services, always check them before attempting to tune the carburettor. Check, too, that the damper tube is topped up with oil to within 1 in. (6 mm) of the top of the rod. Use ordinary multigrade engine oil or the special SAE 20 oil made for the aim by Zenith (the makers of Stromberg carburettors). Tune with the engine running at working temperature.

Adjusting the gap

The fast-idle stop screw opens the throttle when the choke is pulled out. There must be a niche between its head and therefore the choke cam when the choke is pushed home. The CD and CDS have different cold-starting devices, but the adjustment works within the same way. Set the gap consistent with the car-makers recommendations – usually about 1mm. Some Strombergs have a two position screw to limit the utilization of the choke consistent with the season. it’s a spring under its head. Set it with the spring compressed for summer and therefore the tension released for winter (only necessary in freezing weather)

Tuning the CD, CDS and CD2S

There are three adjustments on CD, CDS and CD2S carburettors: a throttle-stop screw, a jet-adjusting screw and a fast-idle stop screw. Check that the choke is fully shut which the fast-idle stop screw is obvious of the choke linkage . Hook your finger under the sting of the dashpot and press the lifting pin upwards to boost the piston by about 1/32 in. (1 mm). If there’s no pin, begin the air cleaner and lift the piston 1/32 in. (1 mm) with a thin screwdriver. hear the engine note while you are doing so. If the mixture is correct the engine speed should rise slightly for a flash , then return again to normal. If it rises and stays fast the mixture is just too rich. If the engine dies when the pin is lifted, it’s too weak.

Switch off the engine before adjusting the mixture, and make sure the jet needle is central. Remove the air filter , lift the piston and let it fall. If the jet is central, the piston falls with a pointy click. Keep the jet central while you adjust the mixture by taking the damper rod out of the highest of the carburettor and pushing a pencil or soft metal rod firmly down the opening to carry the jet in situ . confirm that the jet remains centralised.

Start the engine and convey it up to working temperature. The mixture-adjusting screw is set centrally within the base of the carburettor on CD, CDS and CD2S models. it’s brass and features a wide slot in it. Although a screwdriver are often wont to turn it, alittle coin is easier. Turn only an eighth of a turn at a time, then wait about 15 seconds for the engine speed to calm down . Lift the pin again and see whether the engine speed alters.

Screw the jet upwards (that is, anti clockwise looking down on the carburettor) to weaken the mixture, or down (clockwise) to form it richer. With the mixture setting correct, the idling speed may now be too fast or slow. for most cars it should be 850-950 rpm – judge it by ear if your car doesn’t have a tachometer (or rev counter). Adjust the idling speed by turning the throttle-stop screw. If tuning fails to form the engine run properly, the carburettor may have cleaning or the air cleaner renewing.

Centralising the jet

Lift the piston in order that the needle is obvious of the jet, and screw the jet adjusting screw up until the top of the jet is simply above the top of the bridge within the carburettor bone. Use a spanner to slacken the massive nut just above the jet adjusting screw by half a turn. That releases the jet in its housing, but allows it to drop slightly. Wind the jet adjuster up again until the top of the jet is level with the bridge. Let the piston fall back gently in order that the needle centralises the jet.

Remove the piston damper and hold the piston down with a pencil or soft metal rod slipped into the damper tube. Tighten the jet assembly. Check several times that the piston drops with a click.

Adjusting a Stromberg CD3

The CD3 carburettor features a fixed jet, and therefore the needle is loosely mounted within the air valve, or piston, in order that it centres itself. You need a special tool to reset the mixture, which is done by altering the peak of the needle within the piston.

The tool may be a long L-shaped hexagonal Allen key which matches inside a thick-walled tube. Its maker’s part number is B20379. To use the tool, remove the dashpot damper and insert the tube in its place. Turn the tube until a pin on its side falls into a squeeze the air-valve shaft. Push the Allen key to rock bottom of the shaft and fit it into the needle adjustment. Before you switch the Allen key, hold the outer tube of the tool firmly; otherwise the air valve can turn and tear the rubber diaphragm.

Tune the carburettor with the engine running at working temperature, within the same way like the CD and CDS. For this carburettor, turn the Allen key clockwise to lift the needle and thereby enrich the mixture and anticlockwise to lower the needle for a weaker mixture. Take care to not turn the adjustment too far, or the needle can come loose from the adjuster. The range is merely about two full turns.

If you discover you’ve got to form quite a small adjustment, avoid losing the needle. begin the duvet of the carburettor and scoop the air valve, holding it by its shaft so as to not damage the diaphragm. Then set the needle during a midway position with the Allen key. The right position is when counting on the needle fitted – the washer round the needle or the groove in it’s flush with rock bottom face of the air valve. Reassemble the carburettor and adjust the needle – ranging from now – no quite one turn either way. After each adjustment is formed , run the engine at about 2,000 rpm for ten seconds approximately to clear the additional fuel which tends to urge into the manifold once you are making an adjustment.

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