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Theory of Operations

(Taken from: Shoemark P., Motorcycle Carburettor Manual, Haynes publishing Group, Newbury Park, CA, 1981; ISBN 0-85696-603-7)

In these carbs there are two fuel systems for controlling the fuel/air mixture at low and high engine speeds.

The low speed system consists of the starter jet, the slow jet, the air jet, and the pilot screw.

In the carb throat on the airbox side of the throttle butterfly is the opening of the air jet. This feeds air into the low speed system to help atomize the fuel as it exits the jet openings. These are located in the throat just under the edge of the butterfly in the fully closed position.

The one just at the edge of the butterfly is the bypass hole and is the opening of the starter jet. The other 3, located slightly on the engine side, are for the slow jet and the enrichener circuit ("choke").

The pilot screw controls the flow of air from the air jet and is used to adjust the idle mixture. At idle, a tiny amount of air passes around the butterfly and draws fuel from the bypass hole. At modest butterfly openings, up to 1/4 throttle or so, the incoming air draws more fuel from the low speed system, with the slow jet providing a larger proportion of the fuel as the throttle is opened.

As the throttle is opened further and engine speed increases, the high speed system comes on line. This consists of the main jet, the vacuum piston or slide, and the jet needle, which is held in the slide with the pointed end in the main jet. When the slide is all the way down, the fat part of the needle mostly closes off the flow of fuel from the main jet. The slide doesn't quite completely occlude the carb throat, allowing a small opening for the mix to flow through at low throttle. As the throttle is opened, the slide at first is held down by its spring.

As engine speed increases, the velocity of the flow of air under the slide and across the air lift hole increases, creating an increasing vacuum in the space above the diaphragm. The amount of vacuum controls the height of the slide, which controls the position of the needle and thus the amount of fuel entering the mix from the main jet. When engine speed is high enough the slide is lifted all the way up and the flow of both air and fuel are at maximum. Thus, above about 1/4 throttle it is the slide that controls delivery of the mix to the engine; the butterfly controls it only indirectly.

At WOT the low speed system is still delivering fuel, but its contribution becomes proportionally smaller as speed increases.