So now, we have air flowing through the carb throat, at some pressure less than atmospheric. If a small port is drilled into the carb throat in this low pressure region, there will be a pressure difference between the throat side (what I will call the inside) of the port, and the side that is exposed to the atmosphere. If a reservoir of gasoline (aka the float bowl) is between the inside of the port, and the atmosphere, the pressure difference will pull gasoline through the port, into the air stream. At this point, the "port" gets the name of a "jet" in the concept of a carb. The more air that the engine pulls through the carb throat, the greater the pressure drop across the jet, and the more fuel that gets pulled in. As noted above, within a range of airflow in the throat, and fuel flow in the jet, the ratio of fuel to air that flows will stay constant. And if the jet is the right size, that ratio will be what the engine wants for best performance. I should mention that I am describing a simple carb with a fixed throat and jet size. Like on most older cars. The CV carbs on a BMW work on the same principles, but they vary the throat and jet size in order to extend the range over which the carb can accurately meter fuel.
This is where things start to get more complicated. As I noted, a venturi/jet arrangement can only meter fuel accurately over a certain range of flow rates and pressures. As flow rates increase, either the venturi or the jet, or both, will begin to "choke"-that is they reach a point where the flow rate will not increase, no matter how hard the engine tries to pull air through. At the other extreme, when the velocity of the air in the venturi is very low-like at idle or during startup, the pressure drop across the jet becomes vanishingly small. It is this extreme that concerns us with respect to starting, idle and low-speed throttle response.
At idle, the pressure drop in a 32 mm venturi is so small that essentially no fuel will be pulled through the main jets. But the pressure difference across the throttle butterfly (which is almost completely closed) can be as high as 25+ mm Hg. Carb designers take advantage of this situation by placing an extra jet (the "idle jet" natch.) just downstream of the throttle butterfly. Because of the very high pressure difference at idle, and the very small amount of fuel required, this jet is tiny. When the throttle is open any significant amount, the amount of fuel that flows through this jet is small, and for all intents and purposes, constant. So it's effect on the midrange and up mixture is easily compensated for.
During startup, the amount of air flowing through the carb is smaller still. At least till the engine begins to run on it's own. But when it is being turned by the starter or the kicker, rpm is in the sub-100 range sometimes. So the pressure difference across the jets is again in the insignificant range. Plus, if the engine is cold, it wants the mixture extra- rich to compensate for the fact that a lot of the fuel that does get mixed with air in the carb precipitates out on the cold walls of the intake port. Now we come to "chokes" and "enricheners". Bing carbs, and most bike carbs, use enrichener circuits. All this really is, is another port or jet from the float bowl to just downstream of the throttle butterfly. Except that the fuel flow to this jet is regulated by a valve that is built into the carb body. At startup, when the lever is in the full on position, the valve is wide open, and the fuel supply to the cold start jet is more or less unlimited. In this condition, the amount of fuel that flows through the cold start jet is regulated just like the idle jet is. When the throttle is closed, the pressure drop across the jet is high, and lots of fuel flows, resulting in a very rich mixture, just perfect for ignition of a cold motor. If the throttle butterfly is opened, the pressure difference is less, and less fuel flows. This is why R bikes like no throttle at all until the engine catches. However, the mixture quickly gets too rich, and opening the throttle a tad will make things better. Just like the idle jet, this cold start jet is small enough that even when the circuit is wide open, the amount of fuel that can flow is small enough that at large throttle openings, it has little impact on the mixture. This is why you can ride off with the starting circuit on full, and the bike will run pretty well-until you close the throttle for the first time, and the mixture gets so rich the engine stalls. The valve that controls fuel supply to the cold start jet allows the rider to cut the fuel available through that jet down from full during startup, to none or almost none once the engine is warm. In most cases, at the intermediate setting, fuel to the cold start jet is cut to the point where the engine will still idle when warm, although very poorly since it is way too rich.
True "chokes" are different. But very aptly named. A choke is simply a plate that can be maneuvered so that it completely (or very nearly) blocks off the carburetor throat at it's entrance ("choking" the carb, just like a killer to a victim in a bad movie). That means that the main, idle, intermediate, etc., jets are all downstream of the choke plate. Then, when the engine tries to pull air through the carb, it can't. The only place that anything at all can come in to the carb venturi is through the various jets. Since there is little or no air coming in, this results in an extremely rich mixture. The effect is maximized if the throttle butterfly (which is downstream of the big main jets and the choke plate) is wide open, not impeding things in any way. If the throttle butterfly is completely closed, the engine does not really know that the choke is there-all the engine "sees" is a closed throttle, so there is little enrichening effect. The engine will pull as much fuel as possible through the idle jet, but that is so small it won't have much effect. So a carb with a choke behaves in exactly the opposite manner as one with an enrichener. During the cranking phase, it is best to have the throttle pegged at WFO so that the most fuel gets pulled in, resulting in a nice rich mixture. But as soon as the motor starts, you want to close the throttle to cut down the effect of the choke. Even that is not enough, and most chokes are designed so that as soon as there is any significant airflow, they automatically open part way. Otherwise the engine would flood. Even "manual" chokes have this feature most of the time.
So. I hope this is all at least somewhat clear. "Enricheners" are an extra, controllable jet that provides more fuel to richen the low speed mixture. The amount of fuel they provide is at a maximum when the throttle is closed, and the airflow is at a minimum. As the throttle is opened, the overall mixture effect goes down. Chokes on the other hand, cut off the supply of air to the carb without changing the jetting in any way. But by their action, they increase the pressure difference across all the jets at low engine speeds, enrichening the mixture. This effect is at a maximum at high engine speeds and throttle openings. Which is why different carb types behave different ways during startup.
I have intentionally left this description somewhat general. I will be glad to try and answer any questions that the text raises.