Here is a link to a article I am working on for jetting with some basic theory..
http://www.oem-cycle.com/forum/viewtopic.php?f=5&t=109I have not posted the article on this site yet because I am still working on various aspects.
I'm sure it could use a good proof read.....
Here is the part about your air screw and pilot jet:
Adjusting your Air screwEnough of the theory, lets get into the meat of the article.
Here is a pic of the carb with labels for ID's.
Note: the air screw is usually located toward the air box side of the carb (Big end).
Why start with the Air screw?
The Air Screw is a fine adjustment tool that works along with your pilot jet for starting, idling and the initial phase of your power delivery.
However, as touched on earlier the pilot circuit affects the complete throttle range.
What, how could that be?
Think about it this way, when you are running at full throttle the main jet is delivering most of the fuel, yet, the pilot is also still delivering fuel.
Sure, it's not delivering as much fuel as the main jet but it still is contributing to the overall total fuel being dumped into the system.
Checking your idle screw first will give you a good indication if your pilot jet is too lean or too rich.
Since the pilot jet effects your entire fuel delivery system from idle to wide open it is important to start here.
Otherwise you will be chasing your tail round and round.
Adjusting the Air screw is simple and straightforward.
What I like to do is verify that my float height is set at the correct level.
If the float height is not set correctly your jetting will be off the entire range from idle to wide open.
As a side note the 90-04 KX500 float height should be set at 15-17mm or .590-.670".
I will cover how to check and adjust your float height in another article.
Once the float height settings are verified I clean my air filter, install a new stock spark plug, put in fresh gas, start the bike up, and ride it until it is fully warmed up.
Adjusting the air screw before it is completely warmed up is pointless as you will not know how it will run until after you warm it up fully.
Also, this procedure must be done at the elevation that you plan on riding it at and preferably under the same temperature and humidity conditions that you will operate at on average.
With the engine fully warmed up and running, turn the air screw all the way in (Clockwise). Don't over tighten it. Turn it so that it just bottoms out as that is good enough.
You may have to hold the throttle on a bit to keep it running or adjust the idle so that it is a few hundred RPM's higher then normal.
Once you bottom the air screw out, turn it back out (Counter clockwise) approximately 1/4 of a turn and then wait about 20-30 seconds so that the engine has time to catch up to that setting.
Keep turning the air screw (Counter clockwise) approximately 1/4 of a turn and then wait 20-30 seconds for the engine catch up, do this until the engine reaches it's fastest idle position.
Do not keep adjusting the air screw out past the point where the engine idles the fastest.
If you do your engine will probably run with a lag or be very mushy.
Now that the air screw is adjusted so that the engine is running it's fastest, shut the engine off and turn the air screw clockwise, counting the turns until the air screw bottoms out again.
The ideal air screw position is 1.5 turns.
If the air screw only turns say 1 turn or less then your pilot jet is too small and is running lean at idle.
Installing a new pilot jet is easy.
Simply remove the bowl, unscrew the pilot jet, read the numbers on the side of it and get the correct jet.
Now, install the next size larger pilot jet and start from the beginning, adjusting the air screw.
If the air screw turns 2 or more turns then your pilot jet it too big and the engine is running rich at idle.
Install the next size smaller pilot jet and start from the beginning, adjusting the air screw.
Here is a pic of the carburetor opened up:
The pilot jet is down deep in that hole and requires a flat blade screw driver to remove it.
Here is a pic of the pilot jet removed from the carburetor:
See those little holes in the pilot jet?
They must be clean and free of debris.
You should also be able to look down the center of the jet and see daylight through it with no obstructions.
If adjusting the air screw has no effect then most likely there is a plugged up portion of your idle circuit.
Remember what I said earlier? "Your carburetor must be clean inside and out"
That includes all the little passageways so disassembly will be required.
Eventually you should end up with a air screw setting that allows the engine to idle correctly and is between 1 and 2 turns from the bottomed out position.
There again we are shooting for 1.5 turns.