This will give some insight on the carbs...........
First off, there's 2 basic fuel related problems. You either have a rich mixture, or a lean mixture.
A rich mixture is caused by too much fuel compared to the amount of air being used during combustion. Rich conditions can be detected by the engine spitting and sputtering, blurbling, or acting like a rev limiter, rapidly losing and regaining power. In severely rich conditions, you may be seeing black smoke coming from the exhaust. The black smoke you see is actually raw fuel that is not being burnt and is being wasted. By looking at the spark plug, a rich condition can be detected by a black, sooty plug.
A lean mixture is caused by too little fuel compared to the amount of air being used during combustion. Lean conditions can be detected by the engine losing power, yet retaining it's engine speed. For instance, the engine sounds to be accelerating to higher RPMs, yet feels as if it has no power. By looking at the spark plug, a lean condition can be detected by a white, blistered plug.
Secondly, there are 3 basic carburetor circuits: Pilot Circuit, Mid-range Circuit, and Main Circuit. These 3 carburetor circuits can be troubleshooted by knowing the throttle opening they control.
The Pilot circuit is responsible for throttle openings from Idle (0 throttle) - around 1/4 throttle. This circuit consists of pilot air jet(s), the pilot fuel jet(s), a pilot screw (either fuel or air screw), and pilot ports inside the carburetor throat (a.k.a. Venturi).
There are 2 types of pilot screws: a fuel screw and an air screw.
The fuel screw is located on the engine side of the throttle slide in the carb, and controls the amount of fuel that is drawn into the Venturi by the pilot ports. By turning the fuel screw out, you are allowing more fuel to pass the screw, effectively richening the mixture. By turning the screw in, you are restricing fuel, effectively leaning the mixture. Another way to determine whether it is an air or fuel screw is that a fuel screw has a rubber o-ring to keep air from entering the pilot circuit around the screw.
The air screw is located on the airbox side of the throttle slide in the carb, and controls the amount of air that is drawn into the Venturi by the pilot ports. By turning the air screw out, you are allowing more air to pass the screw, effectively leaning the mixture. By turning the air screw in, you are restricing air, effectively enrichening the mixture.
The air jets are hardly ever changed, so we won't go over that. The pilot fuel jet(s) can be changed to bigger (richer) or smaller (leaner), depending upon your problem. A good rule of thumb to use is that if you have to adjust the pilot screw more than two turns either way if it's stock setting, then you need to accomodate by changing the pilot air or pilot fuel jets accordingly.
Remember, the Pilot Circuit is only effective from 0 throttle to around 1/4 throttle. It still functions during the rest of the throttle positions, but it's effect is minimal, and goes un-noticed.
The Mid-range circuit is responsible for throttle openings from 1/4 throttle - 3/4 throttle.
This circuit is controlled by 2 things: the Jet Needle, and Needle Jet (a.k.a. the Main Jet Holder).
The Jet Needle, or needle as many call it, is attatched to the throttle slide, and drops into the Needle Jet. All needles are tapered. Either the Jet Needle is adjustable or it is not. If there are more than 1 grooves for the needle clip to sit in, then it is adjustable. By raising the clip on the needle, you are allowing the needle to sit deeper into the needle jet, which restricts fuel, effectively leaning the mixture. By lowering the clip on the needle, you are raising the needle out of the needle jet, which allows more fuel to pass, effectively enrichening the mixture.
When the slide raises, it raises the needle out of the needle jet, allowing fuel to pass by the needle and into the Venturi. This is where needle taper comes into play. Unless you are extremely fine tuning the carb, you don't need to worry about taper. You change which part of the taper is in the needle jet by the position of the clip.
Remember, the Mid-range circuit is only effective from 1/4 throttle - 3/4 throttle. None of the other circuits have a drastic effect on this circuit, so if your problem is in the mid-range circuit, then it can't be the main jet or the pilot jet.
The Main circuit is responsible for throttle openings from 3/4 throttle - Wide Open Throttle (you'll see me refer to this at WOT later on).
This circuit is controlled by 2 things: the Main Jet, and the main air jet. The Main Jet is the #1 thing that people change in a carburetor when it comes to tuning them. This is often a big mistake, as it only controls 3/4 - WOT, and NOTHING ELSE. Remember that. A larger main jet will allow more fuel to pass through it, effectively enrichening the mixture. A smaller main jet will restrict fuel, effective leaning the mixture. With the main air jet, it allows air to premix with fuel as it goes up into the Venturi.
The Main Jet only functions at 100% when the slide is open and the jet needle is pulled completely out of the needle jet. At this time, the only thing restricting fuel flow into the Venturi is the size of the Main Jet.
Now for tuning.
If you read above, you should know the difference in feel of rich and lean mixtures. By knowing at what throttle opening the problem is occuring at, you can figure out what circuit the problem is occuring at.
If it's the pilot circuit, there are 3 basic way to tune the circuit. You can adjust the pilot screw, change the pilot air jet, or change the pilot jet.
Adjusting the pilot screw is simple. With the engine running at idle, warmed up to normal operating temps, turn the screw in until it starts to idle rough, then turn the screw out until it starts to idle rough, then turn the screw so it's between those two extremes. To check the position of the screw, you can count the number of turns as you turn the screw in until it seats SOFTLY with the carb body. Reason I capitalized SOFTLY is that the screws (especially the fuel screws) are easily damaged if over tightened. So screw them in until they SOFTLY seat the carb body. Compare your counted number of turns to soft seat and compare it to stock settings (stock settings are determined by counting turns until soft seat before you do any adjustments whatsoever). Again, if you had to turn the screw more than 2 turns either way, you need to change pilot jets (air or fuel) accordingly.
In the mid-range circuit, there are 2 basic ways to tune the circuit. You can adjust the jet needle, or change the needle jet. Raising the clip will lower the needle, leaning the mid-range. Lowering the clip will raise the needle, enrichening the mid-range. You can also change the needle jet, but only if your jet needle adjustments make no difference in the way the mid-range circuit operated. If you are running lean on the mid-range, and you've raised the needle as far as it will go and it doesn't get any better, then you should go up in the needle jet size. Many carb manufactures don't have different sized needle jets, so the aftermarket may offer them, or they may not.
In the main circuit, there are 2 basic ways to tune the circuit. You can change the main jet, or change the main air jet. Changing to a larger main jet will effectively enrichen the circuit. Changing to a smaller main jet will effectively lean the circuit. You can determine which you need to do by first determining whether you are rich or lean. Changing main air jets, again, is for very fine tuning. Once you have the main circuit functioning properly, you shouldn't have to worry about the main air jet, because the air for the circuit is mostly provided by the air passing through the Venturi. On many carbs, the main air jet is not changeable. They may be pressed in.
So there you have it. I basically touched base with carburetor internals and how to adjust them to tune the carb. Every brand carburetor has different ways of accomplishing the same main goal of every carburetor. That goal is to precisely and efficiently mix air and fuel in the right ratios for efficient engine operation. This efficient operation comes from complete combustion, which cannot occur if you are too rich. Whether Mikuni, Keihin, or whatever, they all do the same thing, just in different ways. Hopefully this will help some of you to understand the functions of the carburetors internals.