An engine is basically an air pump: the more air it can pump the more power you get. But it must maintain the right A/F mixture while doing this, which is not easy. In a carbuerator, the A/F mixture is determined primarily by the vacuum in the throttle body. The more vacuum there is, the more fuel is sucked up the jets.

Everyone knows Bernoulli's laws of flow, which basically state that faster moving air is at lower pressure than slower moving air. Volumetric efficiency or VE measures how much volume of air will be moved by a given pressure differential. High VE means a slight pressure differential moves a lot of air -- it's unrestricted. Low VE means things are restricted so it takes a lot of pressure to move a small amount of air.

For example, consider how a choke operates. It restricts the throttle bore to a tiny hole. This restriction means we get a small amount air flow at high velocity. High velocity means low pressure. Low pressure sucks more fuel up the jet. So we have less airflow with more fuel: that means it runs richer.

Now consider a basic guillotine carbuerator. It has one throttle plate. When you open the throttle, the plate lifts. When the plate lifts, the bore is less restricted, enabling more air flow. Given a fixed geometry, more air flow means more velocity, which means more vacuum. That means more fuel sucked up the jets. That means more air and more fuel, so we maintain a relatively constant A/F ratio, which is good.

But all is not good because the above assumes steady state. If you're cruising along and you snap the throttle open, the plate lifts, exposing the bore. What we've actually done is eliminate restriction, which increases VE. High VE means the same volume of air flows with less pressure differential. That means there is less vacuum in the bore. That means less fuel is sucked up the jets. This means we have a transient lean condition until the air starts flowing and it reaches steady state. This will be felt as throttle hesitation.

What happens when you snap the throttle open is that the air in the bore slows way down, which increases the pressure and it draws less fuel up the jets. What we need is a way to keep the air flowing even when the throttle is snapped open. That is precisely what a CV carb does (now you are probably realizing what "constant velocity" means).

On a CV carb there are two throttle plates, one downstream, the other upstream. The downstream plate is hooked up to the throttle. The upstream plate is not hooked up to the throttle. It is a slide that is closed at rest and blocks about 1/2 the bore when closed. The slide is held down by a small spring. There is a small passage from the bore to a vacuum chamber above the slide. As pressure in the bore drops (vacuum in the bore), the vacuum in the chamber works against the spring to lift the slide.

So here's what happens when you snap the throttle open on a CV carb. The small, downstream plate snaps open instantly but the slide remains closed. Thus more air can flow, but since the slide is closed the bore is still mostly blocked. This keeps the bore restricted, preventing a sudden drop in velocity. Maintaining the velocity of air through the bore maintains the low pressure and ensures that fuel is drawn up the jets, preventing a transient lean condition. As the engine revs up and airflow speeds up, the vacuum increases. Increasing vacuum works against the springs and the slide begins to lift. But the slide will never lift too far, else the velocity would drop, increasing pressure, which lowers the slide. It is a self-correcting negative feedback situation. Eventually, when the engine revs up high enough, the slide is lifted all the way and we are running "full bore".

It is absolutely KEY that the vacuum chamber feed passage is an inch or so upstream of the downstream throttle. Thus the slide position is based on the vacuum as measured upstream in the bore -- NOT the vacuum at the downstream throttle plate.