The following writeup on jetting was done by a friend of mine, he is known as sparky660 on Highlifter and ATVCanada. I know it's long but it has a ton of great info.
"First off there are 3 circuits in your carb that relate to THROTTLE POSTION and not RPM. The first circuit is idle to 1/4 throttle that is controlled by the A/F mixture. The next circuit is 1/4 to 3/4 throttle which is controlled by the needle and the last circuit incase you haven't figured it out is 3/4 to wide open throttle (referred to WOT from here on in) is controlled by the main jet. All these circuits overlap and deliver fuel at the same time when at WOT only. When at idle the primary circuit is the only thing that delivers fuel, at 1/2 throttle the primary circuit and needle valve meters how much fuel is coming through the main jet. At WOT the metering rod is fully removed from the main jet and the full amount of fuel is being delivered. If you are lean or rich after changing the main jet and you adjust the A/F mixture screw you will gain very little if any. This is because the pilot jet is so small that the amount of fuel it meters is miniscule compared to the main jet. The proper way to rejet is as follows:
1. Do a plug chop at all throttle positions to begin with and after doing each adjustment a plug chop should be done. Do not go in and change everything at once as you will not have a clue what you screwed up if the thing runs like a POS. If you find that the plug looks good then you have saved you a crap load of work. Use a new plug for each position and for starters do idle, 1/2, and WOT with the engine under load. A plug chop is running the engine and shutting the engine off at that rpm, removing the plug and seeing if you are rich or lean.
2. You find that you are rich or lean after the plug chop so you must change the main jet to smaller or larger depending on the plug read. Do WOT throttle runs until you find the proper jetting.
3. Now that you have the main jet figured you can go to the ½ throttle position. If you are rich hear you move the washer on the needle up so that it lets less fuel through the main jet and opposite if you are lean.
4. Now it is time to do idle. This is fairly easy as you have the engine idling and adjust the idle screw in or out to attain the best idle. The screw either meters fuel or air depending on what carb you have. As on the Grizzly it meters fuel if I remember correctly and you screw it in to lean it out or screw it out to richen it up.
5. After all this you have to go back and do plug chops at the same throttle positions to make sure that you are not rich or lean. If you change any of the carb settings you have to go through the same sequence to see where you are at as each circuit will have a slight influence on each of the other 2. When you have it all done you are good to go. It is also better to be a tad on the rich side as if you are slightly lean any vacuum problems can lead to a really lean condition and melted piston.
This is quite lengthy but to do it properly and gain the most HP out of the engine just throwing in a jet will not help at all unless you are extremely lucky and nail it with just a jet change. The difference between the machines that always seem to run a little faster than most are the ones that have had the carbed tune by someone who knows and understands the carb and what changes influence the machine."
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