Overhaul of these carburetors keep coming up here and there is some very good advice if you search for it. This is a "how does it work" more than a "how do I get it to work" question.
Two years ago there was an excellant post here from Bob M. describing how those ridiculous spring linkages work. I think this link might work:
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someone go a step further and describe how the throttle butterfly controls engine speed? (I have always associated butterflies with chokes and was surprised to find them used at the "other end".) I can see that it can stop air/petrol reaching the cylinder but does it also somehow control the petrol that is sucked into the carburetor? e.g. by reduced air/petrol flow reducing vacuum sucking petrol through "needle"?A bit of knowledge can save so much work . . . Robert