The following was done on a 2001 CR250 Dirt Bike
First, a VERY brief explanation of how a carburetor works might be helpful for some. I won't try to do a comprehensive explanation, just enough to understand why jetting is so important. and what effect jetting has on a carburetor.
The purpose of the carburetor is to provide the combustion chamber with the proper ratio of fuel to air or the"Stoich Mixture". Although this ideal mixture is impossible to reach at all times, the different functioning parts of a carburetor work at all engine speeds and loads to try to create this mixture to the engine for a proper burn.
The parts of the carburetor always include a main jet, fuel bowl, fuel shut-off needle, floats, some sort of mixture screw, and an idle adjustment. The fuel enters the bowl from the tank through a small passage until the floats float high enough in the bowl to force the fuel shut-off needle into the passage and shut the fuel off to the bowl. Usually somewhere in the bottom of the bowl resides the main jet. fuel is sucked through this main jet, and mixed with the incoming air that is creating the vacuum or low pressure effect in the throat of the carb. The mixture screw is able to adjust the air in small amounts in order to fine tune the mixture usually at idle. Some, very few, carbs only require adjustments to some sort of screw to obtain the correct mixture. Most require this screw, and a jet change. When you change a jet, you are simply giving the fuel a smaller or a bigger hole to travel through, thus changing the amount of fuel that is mixed with the air.
The carburetor on most 2-stroke dirt bikes consist of a few more parts than your average carb. Here is a list of the parts involved in jetting a bike and short explanation of the task of each part.
1- Fuel bowl - holds the proper amount of fuel to feed the jets
2- Shut-off needle - shuts the fuel off to the bowl when the proper level is reached
3- Floats - Floats up to force shut-off-needle to closed position at proper level.
4- Main Jet - provides proper fuel at high rpm, above 1/2 throttle
5- Pilot/Slow Jet - provides proper fuel at idle to approx. 1/4 throttle
6- Needle Jet - passage to which the jet needle resides
7- Jet Needle - regulates the flow of the main jet as the throttle is opened, 1/4 to 3/4 throttle.(raise the needle/drop the clip to richen, drop the needle to lean)
8- Air screw - adjusts the amount of air to the idle circuit to 1/4 throttle(in=less air/richer, out=more air/leaner)
9- Throttle stop, or idle screw - adjusts the idle speed 
The first thing to do when working on your jetting is to make sure all of these parts are working and correctly adjusted. If the floats are not adjusted properly, for example, the fuel would be to high or too low in the bowl creating more or less force to the jets, and a rich or lean mixture. Any part that is not in perfect working order will just cause frustration and variation in your readings.
Other things that affect Jetting include elevation, temperature, humidity, aftermarket parts and upgrades, condition of air filter etc.
The main reason jetting is needed is usually because of changes in elevation. At sea level the jetting is going to be quite a bit different than at 5,000 feet because of the amount of oxygen contained in the air at each elevation.
OK, ON TO THE JETTING. Here is something I found on the web that helped me out.
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"golden rules of jetting" :
1) The factory know what they're doing (if you're a long way from standard jetting, something's wrong) This does not apply to RMXs.
2) Most problems that feel like jetting, aren't.
3) Always jet the pilot first, then the main, then the needle.
Pilot jet
OK, after checking the float height is ok and the air screw is in the factory position, start the bike and warm it up, a rag on each radiator helps.If the pilot is too lean the bike will takes ages to drop into a low speed idle after the throttle is blipped. If it's too rich it'll go " gung gung gung" when closing the throttle while rolling in gear.Try to get it as lean as possible with an immediate drop to idle. A good indicator of a too rich pilot is that the choke can be pushed in almost straight away.Dont try to tidy up the pilot with the air screw, any more than 1/4 turn away from the standard setting at the air screw will affect the carbs' operation
Main jet
Jetting the main is usually done by plug chop, ride the bike at full revs loaded in say 4th gear,hit and hold the kill switch in till the bike rolls to a stop.Look for a milk coffee colour on the porcelain, use a used plug that has already coloured up, (note KX85s and 100s will be a lot leaner than the plug chop looks, be careful!)Grey or white plugs are too lean, black coffee is too rich.
Jet Needle
Most bikes wont need the jet needle changed, generally bikes with shorter mufflers than stock will require the needle to be richened by one clip. (raise the needle,drop the clip) this will only richen the bike as it comes onto the main, giving a nice transition and not a wild hit.
Other things
On older bikes make sure the slide and body are a nice fit, not worn and make sure the needle has no freeplay in the needle holder. Remember, FMX jetting is about a nice rip through the mid-range as you hit the up ramp not max HP. I jet Cody Mackies YZ250 for max hp but I jet Maddos CR250 to work any where any time, so it's little rich with a hot plug. Lastly, oil/fuel ratio has little to do with rich or lean jetting. Quality fuel and oil comes first. (taken fromfmxaustralia.com/forum )
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Here's a couple other things I found
1- You don't have to take the carb all the way out to change the jets. Usually you can just remove the top along with the jet needle and throttle cable, loosen the clamps on the boots, and twist the carb inside the boots to get to the bottom of the carb. This is helpful because you will probably need to experiment with many jets.(plan on having a lot of brass left over)
2- A slightly used plug really does come to color quicker. I was using a new one at first and had a hard time reading the plug. Do the check exactly how is outlined above on the main jet by killing it and rolling to a stop, or you will get a bad reading. Mine was used for 2 days before the check.
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This is the setup I ended up with. It seems to do well at 4,250 feet
Main Jet - 360
Slow Jet - 20
Jet Needle on top clip
Air Screw - 2 turns out
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Modifications to the bike include the following:
- mo-better porting job
- FMF Gnarly Pipe
- FMF Shorty Silencer
- Boyesen dual stage power reeds
- UNI air filter
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Finally, here are some good rules of thumb for jetting for different conditions.
Higher temps=lower air density=richer condition
Colder temps=higher air density=leaner condition
Moist air=lower air density=richer condition
Dry air=higher air density=leaner condition
Higher altitude=lower air density(less pressure)=richer condition
Lower altitude-higher air density(more pressure)=leaner condition
So Hot, Moist, and High would need leaner jetting
Cold, Dry, and Low would need richer jetting
and if you have offsetting conditions it would just depend on how much effect each has.
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I am still experimenting with different jet needle positions to lessen the "hit" but just having the correct jetting has reduced the hit significantly and made it easier to handle. The power rolls on very nicely now. I am a little worried about how lean I had to go, but the plug looks perfect right now. I will continue to check the plug to make sure it doesn't get too hot and burn the motor up. Since I did the jetting on a relatively cold and moist day, I may be ok on a hot and dry day, we shall see.
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