If fuel is gushing out of the vent tubes on the top of the carb, that means that your needle and seat assembly is not working correctly. This is commonly called ' float is stuck'. To replace the needle and seat assembly, remove the top of the carburetor by first removing the little clip holding the choke flap arm onto the choke linkage. It is shown (sort of) in the third picture. It is that tiny little clip just to the left of the dirty brown plastic thingy. Next, remove the four screws from the top of the carb, along with the stud that holds the breather assembly wing nut on and remove the top of the carburetor. What you will see is shown in picture #11. In picture #10 you see the float clip popped loose. pop the clip loose on yours and lif the float out of the carb. There will be a little brass thingy with a rubber tip hanging on the end of it, as shown in picture #9. You will then remove the brass seat that is screwed into the carburetor, shown in picture #8. Use a 6 point 3/8 socket wrench for this. When you install the new one, be sure to put the new gasket underneath the seat assembly. Replace the old needle ( brass thingy with rubber tip) , reinstall the float with the needle inside the seat, without dinging it up, reattach the clip holding the float in, and again, you are good to go.
Autolite carbs, with the exception of the K code 4bbls, have thermostatically actuated chokes. How the choke is adjusted is by loosening the three screws on the choke thermostat housing, which is the round black plastic thing on the passenger side of the carb, shown in picture #3, and rotating it slightly, then tightening up the screws. You will notice that there is an indexing mark on the carb to help you keep track of the adjustments you make. Rotating the housing clockwise makes the choke open sooner, counter-clockwise makes the choke open later.
The idle speed is adjusted by running the screw on the throttle linkage on the driver's side of the carb, as shown on the right hand side of picture #12. The fast(cold) idle speed is adjusted by the screw on the choke linkage over on the passenger side of the carb shown kind of right in the middle of picture #14. Nothing to it.
Here's how your Autolite 2100 or 4100 carburetor does what it does. Understanding this is essential to diagnosing and correcting any problems that it might be having. The carb is responsible for delivering fuel to the engine. How it does this is by a couple of basic physical principles. One is that air moving through an hourglass shaped tube is at it's highest velocity and lowest pressure at the skinny part of the tube. This was discovered by an Italian physist named, that's right, you guessed it, Giovanni Battista Venturi. Your carburetor's fuel bowl has a vent hole on top of it. The pressure at the top of the fuel is normal atmospheric pressure because of the vent hole. The fuel has a few different routes that it can take to get into the venturis, but at the discharge port, the air pressure is lower, because of the venturi effect, than it is at the top of the fuel bowl. That creates a 'push' on the topside of the fuel. An Autolite carb has three basic fuel circuits. One is the idle circuit. This is the path that the fuel follows when the throttle plates at the bottom of the venturis are closed. The discharge ports for the idle circuit are below the throttle plates. This works by straight-up suction, or engine vacuum, so that when the car is at idle speed, fuel is delivered to keep that going. Next is the transition circuit. Here, the discharge ports are slots that are partially below and partially above the throttle plates and works partially by the suction of the engine and partially by the pressure differential of the venturi effect. Last, we have the main fuel circuit, which works entirely by the venturi effect. The discharge nozzles for the main circuit are the ones that are inside the booster venturis, or, the little hourglass tube shaped thingies that you see inside the throat of the carb. The float that is inside the fuel bowl is responsible for keeping the fuel at a certain level. The correct level for the fuel in bowl is slightly below the main discharge nozzles. If the level is too low, the venturi effect won't be strong enough to push the fuel uphill and get it into the carb. If it is too high, gravity will pull the fuel out of the fuel bowl until the two levels are the same, and then the siphon effect takes over and drains the fuel bowl entirely. Where that fuel goes, when the motor isn't running, is straight into your oil pan, degrading the lubricating ability of the oil and making you have to crank on the starter until the fuel bowl is filled up again so that the car can start. You also have an accellerator pump to deliver a little shot of gas to keep the motor running while the carb changes from one circuit to the next. This is also what the power valve does. It is held closed by engine vacuum. As you open the throttle plates on the carb, engine vacuum drops temporarily, allowing the power valve to open and let some more gas into the engine after it has burned the little shot provided by the accellerator pump. The engine vacuum recovers pretty quickly and the power valve closes and the motor is running on the main fuel circuit entirely at high speed cruising speed. Normally the motor is running on a combination of two circuits, depending on what you are asking the car to do, and most round-town driving is down on the transition circuit. Puddles of purely liquid fuel do not burn very efficiently at all. That is why the carb is designed to vaporize the fuel, and you hear people talk about the ' air/fuel mixture'. Inside the carb there are these little thingies called emulsion tubes. These create a mixture of highly airated, but still basically liquid, fuel. When this air fuel emulsion goes out the discharge nozzle, the process of vaporizing the fuel is finished by the shearing and velocity change of the mixture, so that it is vaporized as effectively as possible when it gets into the motor. On the four bbl carbs you have two primary venturis and two secondary venturis. The 4100 has vacuum secondaries. What that means is that the change in engine vacuum caused by hard accelleration or wide open throttle is what causes the secondary venturis to open, allowing even more fuel into the motor, much like a great big power valve. And that is how that works.