Fixed choke carburetors require many different and circuits in order to maintain a fuel supply that is in step with the constantly changing vacuum in the venture. Variable choke carburetors avoid this by altering the size of the airflow. This results in a constant vacuum in the venture and it is only necessary to have one fuel jet, the size of which varied to provide the correct mixture for all operating conditions.
Variable choke carburetors are usually of horizontal design. They have a conventional float chamber, but the single outlet from this leads to a jet mounted in the lower part of the barrel, in the center of a raised section which partially obstructs the airway.
The upper part of the has an enclosed housing or suction chamber containing a two-diameter or air . In its lowest position, when the is stationary, the narrow base of the rests on the raised section in the highest position, it unblocks the barrel completely, allowing unobstructed airflow. The is hollow and small drillings in its base on the side permit air to enter and leave the suction chamber above its large upper diameter.
When the is started, the obstruction of the creates a vacuum in the side of the barrel. The holes in the transmit this vacuum to the suction chamber above the larger diameter of the , which causes the to rise. The amount of lift depends on airflow, and the vacuum acting on the jet which always remains constant.
To vary the amount of fuel flowing into the carburetor barrel, there is a tapered needle, attached to the base of the piston and moving vertically in a petrol jet. At low engine speeds, when the piston practically the barrel, only a small quantity of fuel emerged to mix with the proportionally small airflow. As speed increases, however, and the rises, the obstruction in the jet becomes progressively less and more fuel flows out to mix with the extra air.
The taper of the needle is designed so that the ideal mixture can be supplied at all speeds. This type of does not need a separate idling circuit, but provision for cold starting is provided by the linkage which lowers the jet from the needle, thus giving a mixture.
As with a fixed choke carburetor, the variable choke design needs enrichment for acceleration and this is provided by a hydraulic damper which resists sudden upward movement of the piston when the throttle is opened. This brief delay allows extra vacuum to act on the jet and draw out a richer mixture.
There are two basic types of commonly in use. The main difference between them is in the way in which an air seal is provided around the rim.
One has a which is a very close fit within the suction chamber, whereas the other uses a flexible diaphragm clamped around the top and the rim of the suction chamber.
The Economic Circuit
A cruising can accept a weaker mixture than an running under heavy load. To exploit this, economic devices are used. A common layout is a spring-loaded diaphragm which obstructs a channel leading to the main jet circuit. The above the diaphragm is connected by a drilling to the barrel below the throttle. When the is cruising, the increased vacuum below the throttle lifts the diaphragm against spring pressure, unblocking the channel and bleeding extra air into the main jet circuit to weaken the mixture. If the throttle is opened for additional power, the vacuum below the throttle falls, the diaphragm again the channel and the main jet supplies a richer mixture.
An alternative way is to use the diaphragm to open and close an additional fuel supply running to the main outlet. The principle is similar, the diaphragm being operated by manifold vacuum, but the supply is obstructed by cruising and extra fuel is allowed to flow to enrich the mixture on wide throttle openings.
Originally posted 2018-09-11 15:21:17.
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