First up, always have it in your head that the carb is reacting to the vacuum produced from the piston moving down the cylinder bore, as that vacuum is produced pressures change inside the carb and fuel is sucked from the bowls as a result. Where that fuel comes from, and how much is delivered is determined by the jets/needles/screw settings etc.
The first diagram is a CV carb, two points to note, the starter enrichment circuit (its not a true choke by the way) is not shown, but if people want to know how it works i can make a separate diagram for it, and secondly this picture shows the pilot and main air jets in the body of the venturi, whereas in our bikes its parallel to the venturi, positioning aside this makes no difference to the functioning of the carb.
![Image](https://farm2.staticflickr.com/1624/24470890272_e7d10a6e3b_b.jpg)
When you hit the starter button you create a vacuum on the engine side of the carb that is at a lower pressure than the atmosphere on the other side of the carb. This causes air to be sucked through the main body of the carb. This low pressure on the engine side causes fuel (RED) to be sucked from the float bowl through the pilot jet, mixed with air from the pilot jet (BLUE) and the fuel air mixture (PURLPE) sent through the transfer ports and the pilot opening via the mixture screw...see the diagram below:
![Image](https://farm2.staticflickr.com/1669/24211453899_25ee698365.jpg)
The purpose of the transfer ports is to ensure that when you slam the throttle closed after being at WOT there is some fuel that can always be found by the engine to prevent you stalling.
You can see therefore that the purpose of the mixture screw is to vary the fuel/air mixture going into the engine, if you turn the screw so it closes fully the only mixture going to the engine is from the transfer ports, by opening the mixture screw fully you allow fuel to pass through the transfer ports and a large amount to go via the pilot opening.
This next picture shows the engine running at max revs with a WOT:
![Image](https://farm2.staticflickr.com/1600/24470889722_8692481f9f.jpg)
Three things have happened between idle and WOT:
1. The butterfly valve has moved into its least restrictive position.
2. The increased vacuum present in the carb has created a vacuum (GREEN) in the slide chamber, the rubber diaphragm has been sucked up onto the roof of the slide chamber as air has been sucked via the slide port, this has the effect of raising the needle jet and in doing so creates a larger "hole" in the end of the main emulsion jet.
3. The lower pressure in the carb is causing fuel and air to be sucked from the main jet and from the main air jet respectively, these are mixed in the main emulsion tube, flow past the needle jet and out into the carb.
A diagram of the fuel flows is shown below:
![Image](https://farm2.staticflickr.com/1595/23951061944_e6a5aa0e98.jpg)
This rpm % on the x-axis, and fuel required by the engine (as a percentage of the max available on WOT) on the y-axis, you can see that the pilot jet meters a near constant amount of fuel that is good for tickover, as engine revs rise more fuel is required by the engine, and this is delivered from the main jet.