HERE'S OUR GUIDE TO PROPERLY TUNING YOUR CARBURETOR
A correctly jetted carb makes a tremendous difference in the torque, midrange pull, top-end pull, and over-rev of your engine. If you have never jetted your bike correctly, you will almost certainly gain some performance at some point in the bike's powerband. A cleanly jetted pilot circuit can be the difference between having to clutch the bike out of a turn or not. The needle can make all the difference in the world for the power of the machine in most situations, as it controls the throttle range that most riders spend most of their time using. A correctly sized main jet could mean the difference between being able to rev out high enough to not have to shift one more time at the end of the straight, or the power falling flat on top and requiring you to make that extra shift. Are you fouling plugs? Many people will tell you all sorts of band-aid fixes, from running less oil, to running a hotter plug. Both are incorrect fixes for plug fouling. It's all in the jetting. The only way to know what jetting changes you will need is by trial-and-error. No one can give you jetting specs, because every bike is different, every rider has a different style, and jetting is totally weather dependent. Unless the person telling you what jets to use is riding an identical bike, on the exact same track, at the same time, his recommendations are meaningless. Jetting is fairly simple, and is a useful skill to learn if you want it to perform at it's best.
Pilot Jet:
It's very important that you start with the pilot circuit. The reason is simple. The pilot circuit affects the entire throttle range. When you are at full throttle, the main jet is the primary fuel metering device, but the pilot is still delivering fuel as well, adding to the total amount of fuel that your engine is receiving. Before you start to rejet your bike, you need a clean air filter, a fresh plug (actually you need several plugs to do plug-chop tests for the main jet), and fresh fuel. One important detail: Make sure the engine is in good mechanical condition. If your engine has a worn top-end, fix it first. Trying to jet a worn out engine is a waste of time. The same goes for reeds that don't seal properly, and a silencer that needs re-packing. Worn reeds will mimic rich jetting, and worn rings will mimic lean jetting. Before you start the jet testing, install a fresh plug. Set the float level to the proper specs, an incorrect float height will affect your jetting all across the throttle range. Warm the bike completely, and shut it off. As already stated, start with the pilot circuit. Turn the airscrew all the way in, then turn it out 1.5 turns to start. Start the engine, and turn the idle screw in until you get a slightly fast idle, or hold the throttle just barely cracked, to keep the engine idling. Turn the airscrew slowly in, and then out, until you find the point where the idle is fastest. Stop there. Do not open the screw any farther, or your throttle response will be flat and mushy, and the bike may even bog. This is only the starting point, we will still have to tune the airscrew for the best response. Now is the time to determine if you have the correct pilot installed in your carb. The airscrew position determines this for you, making it very simple. If your airscrew is less than 1 turn from closed, you need a larger pilot jet. If it is more than 2.5 turns from closed, you need a smaller pilot jet. Once you have determined (and installed it if it's necessary to change it) the correct pilot jet size, and tuned the airscrew for the fastest idle, it's time to tune the airscrew for the best throttle response. Again, make sure the bike is at full operating temperature. Set the idle back down (the bike should still idle, despite what you read in the Moto Tabloids), and ride the bike, using closed-to-1/4 throttle transitions. Turn the airscrew slightly in either direction until you find the point that gives you the best response when cracking the throttle open. Most bikes are sensitive to changes as small as 1/8 of a turn. The airscrew is not a set-it-and-leave-it adjustment. You have to constantly re-adjust the airscrew to compensate for changing outdoor temps and humidity. An airscrew setting that is perfect in the cool morning air will likely be too rich in the heat of the mid-day.
Air/Fuel Screw:
Tuning your Air or Fuel screw. What's the difference between the two? Easy to tell, If the adjustment is on the airbox side of the carb it's an Air screw. If the adjustment is on the engine side it will be a fuel screw.
Start by setting the mixture screw to the stock setting your machine calls for. Don't know this setting? Thats fine. Most 2 strokes are 1½ Turns out from lightly seated. Most 4 strokes are 2¼ Turns out from Lightly Seated.
Now, Warm the machine up. This needs to be preformed with the machine warm, at idle and the choke off.
Slowly bring out your mixture screw. The idle should start to rise. Keep slowly bringing the mixture screw out as the idle get's higher and higher just until you hear the idle start to come back down. Then turn it back 1/8th turn and re-adjust your idle back down.
Didn't notice a change in the idle? If you have brought the mixture screw out more than 3 turns and there was no change in idle, Take it back to your starting point and do the exact same process but in reverse. Go in with your fuel screw instead of going out.
Once you have this set you can ide he machine and fine tune with +/- 1/8th turn at a time to get the best off idle throttle response.