Throughout this site we use many technical terms, and given the breadth of readership our site enjoys, sometimes we are remiss and incorrectly assume everyone knows what we are referring to. For those that do not, here are some explanations of the technical terms use. |
Idling System
On fixed-choke carburettors, as the throttle butterfly is progressively closed and engine speed falls, air flow through the venturi (choke tube) reduces to a point where normal fuel delivery rapidly decreases, giving a mixture too weak to support combustion and the engine stops. To allow satisfactory running at low speeds and particularly when idling, a separate system is built into the carburettor which supplies fuel through an idling jet to one or more outlet holes in the barrel adjacent to the edge of the throttle butterfly in the closed position. The high air velocity past the butterfly in this position causes sufficient depression to draw off fuel/air mixture from the idling system.
One orifice on the engine side of the closed butterfly will be under depression in the idling position, while one or two holes just on the other side will then allow air to bleed into the idling supply duct. These latter are known as "progression orifices" or "transfer ports" because as the butterfly is open they, too, come into the area of depression and become fuel outlets so tending to enrich the mixture as the engine speed is accelerated. On later constant vacuum carburettors a separate idling system, previously considered unnecessary with a variable choke, may now be found. |
