How Compound and Twin-choke Carburetors operates
Modern engines are required both to operate at motorway speeds and to perform satisfactorily in heavy traffic.
On some engines, it is difficult to provide a carburetor which is large enough to supply sufficient air for high-speed running, yet small enough to maintain the operation of the main circuit correctly at low speed.
For this reason, compound carburetors that have two or more barrels side by side, have been developed. These are of the fixed choke type and, in effect, are two carburetors in one.
At lower speeds, opening the throttle works only one barrel, the second barrel remaining closed. The venture action within the first barrel provides sufficient vacuum to operate the main circuit efficiently. At higher speeds, where the small diameter of the first barrel would limit engine power, a vacuum-operated device automatically opens the second barrel.
Alternatively, there is a mechanical linkage which operates the second barrel when the first is open more than a certain amount. These are known as ‘progressive twin-choke carburetors’.
True “twin-choke” or twin-barrel carburetors have two throttles that open simultaneously. They are commonly used on ‘V’ engines, where each barrel supplies one bank of cylinders.
An engine may have two or more separate carburetors, each serving one or more of the cylinders. This arrangement normally produces slightly more power than a single installation because it gives a better distribution of the fuel/air mixture.
With a single carburetor, the fuel/air mixture leaving the carburetor is usually drawn through a multi-branch inlet manifold to each cylinder in turn, resulting in the mixture continually having to change direction and negotiate sharp corners.
In some cases, the cylinders may not all receive the same amount of mixture so that the carburetor has to be set waste-fully rich to feed the weakest cylinder properly.
With multiple carburetors, less complex manifolds are used and where each cylinder has its own carburetor barrel, or one barrel feeds two or three cylinders, the problems of unequal supply and awkward manifold shapes are avoided.
THE EMISSION CONTROL OF INTERNAL COMBUSTION ENGINE
The internal combustion engine pollutes the atmosphere. The chief pollutants are unburnt or partially burnt fuel i.e. hydrocarbon, oxides of nitrogen that are formed at very high temperatures in combustion chamber, and carbon monoxide which is present in the exhaust.
A major contributor to pollution is imperfect carburation, and since the early 1970s carburetors have been redesigned or modified to produce a more accurately metered fuel/air mixture.
Compared with earlier models, these emission type carburetors produce a weaker fuel/air mixture mainly at idling speeds to ensure complete combustion of the fuel.
A weak mixture tends to give an uneven or lumpy tick-over and to avoid this, engines designed to produce a less harmful exhaust are usually set with a fast idling speed than ‘non-emission’ engines.
To discourage private owners from making carburetors adjustments that would upset the inbuilt settings, external adjusters that control mixture strength and engine idling speed are concealed or capped to prevent tampering on carburetors made after 1972.
Currently, no penalty exists for altering the controls on a tampered-proofed carburetor, but it has been suggested that a check of the idle mixture ratio should become part of the annual MoT test for cars that are three years old or more.
If this happens, it is probable that cars found to have harmfully pollutant exhausts will need to have their carburetors reset by authorized agencies.
