Car Parts and Accessories

A morden carburettor fitted with various emission devices does not differ in principle from the compound carburettor. It is usually a fixed jet carburettor with two or more chokes and a number of anciliary components which control the mixture more accurately than a pre-emission equipped carburettor.

The choke is frequently automatic. A chamber on the end of the choke butterfly spindle contains a bi-metallic coil which is connected to the car’s electrical system when the engine is running. It may be wired through an engine temperature thermostat. This permits current to flow when the engine is warm. Current in the bi-metallic strip changes its spring effect on the choke spindle and opens the choke butterfly as the engine heats up.

The primary choke is usually smaller in diameter than the secondary choke. The two may be linked mechanically or, in some cases, the secondary choke’s butterfly may be opened by a vacuum solenoid which senses manifold depression. When there is little manifold depression (at maximum pressure on the accelerator pedal) the mechanical linkage, or the vacuum solenoid allows the secondary throttle butterfly to open increasing the amount of fuel and air that can be admitted to the engine. This increases power when it is needed without the fuel economy penalty and poor emissions encountered with multiple carburettors.

To prevent engine damage when cold, the secondary choke may be prevented from operating by a baffle in the secondary choke. To keep emission levels low on a trailing throttle, the primary choke butterfly may be held slightly open by a solenoid and to prevent running-on when the engine is switched off, another solenoid may be used to cut off the fuel supply to the carburettor’s jets.

Transverse, or east-west, engines (usually front-wheel drive cars) may have a carburettor with the float chamber partly between the two chokes. This reduces fuel surge when cornering.

Monitoring the Carburettor

Carburettors on engines built after the introduction of emission-control regulations are frequently complex. The carburettor’s functions are monitored and modified according to emission and running requirements. Some functions are monitored electronically, others electrically and a few mechanically. Additional emission equipment may control the mixture when the engine is warming up.

The Stromberg CDS Carburettor

THOUGH IT WORKS on the same constant- pressure principle as the SU carburettor, the Stromberg CDS differs in several design features. Instead of having a closely fitting piston, the suction chamber is sealed with a flexible diaphragm which reduces the weight of the piston and ensures a perfect seal.

The float chamber surrounds the jet and is fitted with two floats, one on each side of the jet, mounted on a common spindle. The double floats make this design less sensitive to flooding when the car is tilted. The main jet is spring-loaded against an adjustment screw; and the jet assembly is retained by a housing, and is sealed by ‘0′ rings. As with SU carburettors, alternative needles and piston springs are available for engine tuning.

The mixture for cold starting is provided through a disc valve operated by the choke control. Rotation of the disc, which draws fuel from the float chamber, aligns a series of holes drilled in the disc with a fuel passage into the carburettor body. Operation of the disc is linked by means of a ‘fast idle’ cam to the throttle which is opened a predetermined amount.

This disc starter has two settings—one for summer and the other, providing a richer mixture, for winter.

To prevent over-rich mixtures at idle when temperatures are high, a further valve may be fitted. This is controlled by a bi-metal spring, which reacts to the air temperature, and allows extra air to enter the carburettor, between the venturi and the throttle.

The H-type SU Carburettor in action

A wide variety of variable-jet carburettors are manufactured by SU. They differ only in the method of jet assembly and in the means of feeding the fuel from the float chamber to the jet. The H-type is fairly widely used and is produced in a range of throttle body sizes.

The piston is guided by a bush fitted in the suction chamber, and is a close fit, which is necessary to prevent excessive air leaking past the piston. The piston has a hollow stem filled with oil, with a plunger to act as a damper.

The main air passage is connected to the suction chamber by a passage in the piston, so that air can be drawn from the chamber.

The float chamber in an SU is separated from the rest of the carburettor and is attached to it by a single bolt, either rigidly or with a flexible rubber mounting.

Fuel is fed to the jet through a short flexible pipe. The position of the jet can be altered by sliding it in a long bearing pressed into the carburettor body. Adjustments are made by means of a nut or a remote screw and linkage.

For cold starting, pulling out the choke control on a car’s instrument panel lowers the jet assembly about 9 mm. This ensures a larger fuel discharge area and gives the required rich mixture ; it also opens the throttle slightly.

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