Choke is restricting air intake when the engine is cold with the choke plates on top of the carb. By restricting air flow it lets engine's vacuume get to the fuel passages pulling the fuel from all orifices that normally only feed the fuel at higher RPMs. At start up the mixture is very RICH mixture (lots of gas and very little air) as the cold engine has harder time igniting normal mixture. As the engine warms up, the choke opens up slowly letting more and more air thus making the mixture a bit less rich until finally when it finally opens you have proper mixture (not too RICH and not too LEAN).
Before you start the engine, it's important to press the gas pedal ONCE on cold engine that sat through the night (or on cold days for more than 1 hour). When you do so, the choke engages (flaps on carb's top close). When you turn the key and the car doesn't start right away, do NOT press gas pedal second time to give it more gas as the engine is turning! This will release the choke making the start even more difficult. Just let go off the key so the engine stops turning. Press gas again (maybe twice) to squirt extra fuel into intake manifold and turn the key again. On very cold days it can take 10 plus seconds to start the engine so just keep it cranking. If after a few seconds of cranking the engine is still not engaged, turn the key to the off position and press the gas again setting the choke one more time.
When the engine is finally turning, you will hear picks up RPM then it could start stubling untill again reving higher than your regular idle. If at that time you tap the gas pedal it will release the choke partially and your idle RPM should drastically decrease (if you do it too soon and keep the car parked you might hear the engine stumbling because it's not warm enough).
If you were to look at what happens with the choke you would see:
1. press the gas pedal before starting the car = choke closes completely and FAST idle screw engages
2. engine starts and the vacuum immediately pulls the choke plate slightly open (around 1-4 mm depending on your car and carb type)
3. as the engine warms up, the choke plate opens slowly letting in more and more air
4. if you do not touch the gas pedal the engine will rev up much higher than your regular idle because the FAST idle screw is holding the cam level that controls how much the throttle plate is open. For cold start, the throttle plate is made to be open way more than for regular idle thus the engine is sucking in a ton more gas than at regular idle. If you want to keep on idling at regular speed simply tap (tap, don't floor it!) the gas pedal and it will release the fast idle hold bringing your RPM to regular speed (don't floor it when the engine is too cold or it might stumble - on some carbs such as Solex there are steps on the fast idle cam to prevent throtles from closing fully when floored, on others such as Weber it isn't there).
5. if you were to floor the gas when the choke still hasn't fully opened you can get blow back from the carb so for the first few minutes of driving, before engine warms up fully, be a bit gentle with your gas pedal.
The only three real adjustments of a choke component is FAST idle screw (how much the throttle plate is open when cold - this infulences RPM on cold engine idle); the choke opening right after engine starts (this can't be adjusted easily on Weber and is easily adjusted on Solex); and the choke speed with which it opens the plate. The choke has 3 screws holding it down to carb body. If you need to adjust the speed with which the choke plates open just unscrew the screws and move the choke left or right. One way speeds up the speed, while another slows it down.
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