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1. The Float The float bowl acts as a fuel reservoir to meet engine demand. The float is hinged on a pin in the float boss. It rises and falls with the fuel level in the float bowl. The small metal tang integrated in the plastic float supports the float valve, also known as the float needle. As the fuel in the float bowl rises, the float valve is pushed into the valve seat, until it’s high enough to shut off the fuel flow to the bowl. As fuel is used the level in the bowl drops lowering the float which pulls the float valve from its seat, and fills again. Adjusting the height of the float has a big effect on the mixture as a low or high float level makes it harder or easier for the vacuum to suck fuel into the venturi. Differing float levels cause an imbalance which may be perceived as vibration. 2. The Choke This system is referred to as the choke. But that’s a misnomer. When you pull the choke knob, what you’re doing is retracting a plunger that opens a tube connected to the starter jet, allowing additional fuel to enter the venturi just below the vacuum hose nipple. It supplements the pilot system at start up. 3. The Pilot System The primary purpose of the pilot system is to supply the mixture at idle. It continues to supply fuel throughout the entire throttle range, but after about 1/8 throttle is reached the main system starts to put out more of the total mixture, up to full throttle. By adjusting the idle with the big screw on the left side of the carburettors the position of the butterfly is altered, so exposing one or more of the four small holes that are drilled into the venturi, (leading to the pilot jet) just under the butterfly valve, letting more or less air pass the butterfly. Adjusting the pilot screw that’s under the carburettor varies the amount of air premixing with the fuel before it enters the venturi. 4. The Main System Open the throttle and the cable that’s connected to the butterfly valve turns it from vertical to horizontal, so letting more air through the venturi. This increases the vacuum effect that is transferred up through the vacuum drilling in the slide to the diaphragm valve that leads to the diaphragm chamber. The top chamber is separated from the bottom by a rubber diaphragm. The bottom chamber is open to atmospheric pressure from the airbox. When the vacuum in the top chamber rises enough, the constant ambient pressure of the lower chamber helps the diaphragm valve overcome the downward force of the diaphragm spring, so it rises from the ven- turi. As the diaphragm is raised the needle is pulled out of the needle jet, exposing a thinner portion of the needle taper which allows more fuel to rise into the venturi to meet the increased engine demand. The key parts of the main system are shown in the photo below
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Triumph Bonneville Tuning Manual
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