How do SU (HS2) carbs work? (theory of operation)

[pat@sbctri.sbc.com (Pat Niemeyer)]

I'm in the process of re-building the SU carbs from my '66 Spit, and I
am having a little difficulty understanding the physics behind how these
work...

Let me try to draw an ascii art picture:


                 |Piston    |
    |            |_thing____|           |
    |                 ||                |
    | Chamber         ||N               |
    -----------\      ||e     /---------
               |      ||e     |
               |  |   ||d  |  |
               |  |   ||l  |  |
               |  |   ||e  |  |
               |  |   ||   |  |
               |  |   || . |  |
               |  | . ||   |  |
               |  |   \/ . |  |
               |  | .   .  |  |
               |  |.  .  . |  |
               |  | .   .  |  |
                --| Fuel   |--
                  | .    . |
            Jet ->|   .    |
                  | .   .  |
                  |        |

(obviously the scale and proportions are exaggerated)

What I'm trying to understand is how the relative positions of the needle,
the jet, and the fuel affect the mixture...

Is the piston thing raised directly by engine vaccum?   How does the height
of the needle allow more fuel in?

I can see empirically that the position of the jet is determined by the choke,
so it must be responsible for changing the mixture, but how exactly?

Any explanation would be helpful...


Thanks,
Pat