Hmm, carbs, the good news is this is about the simplest carb around, bad news there isn't one thing to know but rather a lot of things to take into consideration. Let me try to give you a general summary of what is going on in the carb.....
When the piston pulls down it creates a suction which draws air through the carb, this air draws fuel up from the carb bowl through the carb jet which mixes with the air. Tuning this mixture is tuning your carb. When there is too much fuel and not enough air it is called "rich", when there is too much air and not enough fuel it's called "lean". A rich mix may make your motor run poorly, a lean mix may make it run too hot.
There are several factors that come in play here...The size of the hole in the main jet is one major factor, the bigger the more fuel in the mix. The air filter offers some resistance to air flow, more resistance means less air to fuel so it richens the mix. (so keep your air filter clean, cause if it gets too dirty it can richen your mix and make your motor run poorly) The muffler also offers some resistance to air flow as well so a change in the exhaust can also effect the mix. It's a sealed system that must be balanced to make your mix proper, for this reason it is very important that you don't have any air leaks at the carb/intake, intake/cylinder, and exhaust/cylinder junctions. (and at the cylinder head as well for that matter) Your oil/fuel mixture also has an effect on your air/fuel mixture, the oil displaces gas, so more oil will lean out your air/fuel mix.
Inside your carb you have the slide which has the needle inside it. When the throttle is at rest the slide is blocking off the air flow through the throat of the carb and the needle is down inside the main jet reducing the size of the opening and limiting the fuel flow. When you twist the throttle the slide and needle rise opening up the throat of the carb for more air flow and lifting the needle out of the jet allowing more fuel to flow. (the idle screw simply props up the slide a little bit to allow a little air and fuel to flow, its like twisting the throttle a tiny bit) The carbs need to be tuned for three throttle ranges, idle/low, middle and WOT (Wide Open Throttle). I know, seems like it should be simpler, but its not too complicated....
You need to start out tuning for WOT, because the factors that control the WOT mix will effect all throttle ranges. The resistance of the air filter and exhaust and the size of the main jet have the most effect here. The fuel level in the carb bowl will also effect the mix at WOT, so the float setting will affect the mix since it sets the fuel level in the bowl.
Once you have a good mix at WOT you then tune for the mid throttle range. The needle setting has the most effect on mid throttle tuning. The top of the needle will have several grooves in it with a little clip on one of them, moving this clip is how you adjust the needle setting. If you move the clip toward the end of the needle (away from the point that goes in the jet) the needle will set deeper in the jet and therefore reduce fuel flow and lean out your mid throttle mix. If you move the clip toward the point it pulls the needle further out of the jet allowing more fuel to flow creating a richer mid throttle mix.
The final adjustment is the idle/low throttle range. To set this you simply screw in the idle screw to the desired idle speed, and then adjust the air/mix screw to where the motor idles best.
With a new motor you kind-of need to hold your horses a bit on tuning, because the break-in fuel mix effects the tuning and the fact that the rings on the piston haven't set yet. For break in you need to run more oil in the fuel to help get all the moving parts in the new motor lubricated. If you've got your motor running fairly decent and you are running break in fuel then you should let the carb be for the time being and let the rings set and wait until you are on your regular fuel mix before you start tuning your carb.
By now you are probably wondering how the he!! you figure out what you have going on in your carb, rich or lean, or in between? The best way to tell is by doing a "plug chop". To do a WOT plug chop you ride the bike until its warmed up, then ride at WOT for a bit then all at once let off the throttle, pull in the clutch and hold down on the kill switch. (So the motor dies and is disengaged from the rear wheel) Pedal the bike home and let the motor cool and then pull the spark plug and inspect it. (never remove a hot spark plug) To do a mid throttle plug chop do the same while riding at mid throttle....
Here is a link to a very helpful chart that can help you read your spark plug and determine what is going on in your combustion chamber....
http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html
That is a pretty complete general overview, hope it wasn't too complicated... My advice to you would be to make sure you don't have air leaks, make sure your head is torqued down proper, ride through two gallons of break in fuel mix and than analyze how your motor is running at WOT when you are running a standard fuel mix and go from there. Post back at that time and we can help you figure out how to tune your carb up if need be....
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