Lister D carb

I'm a first time owner of a Lister D and I'm unsure the correct proceedure/operation of the carb....in relation to the small spring loaded nob on the air side.I think the carb is a standard one(listers own)(?). Can you help

I think you are referring to the small rounded top shaft that comes out the top of the air side, and is connected to a valve-like plate on the underside, where the air enters?
If so, that regulates the air intake. It rises as air intake increases.
If you hold it down in effect this chokes the carb, and might be a starting means (modern carb has a butterfly for a choke).

Is this what you mean?