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Originally Posted by swazi_matt
....and therefore it is usually best
not to set them to idle. You will find that the idle speed adjustment and the required low speed
mixture setting will change drastically from a warmed up engine in the pits to a hot engine
several laps into the race. It is best to set the slide position (idle speed screw) so that the engine
will return to a low RPM for a few seconds and then die...
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I really disagree with the old 1970's ideology that a 2T shouldn't idle as it's just not true. I agree they are more inefficient at idle but that's the case over the entire rpm range except for what rpm the pipe is specifically tuned to. Every 2T I have ever owned (KDX's, KX's, YZ's, EC/DE's, IT, and RM) would/will run and idle really well all day on my initial air screw adjustment as long as I'm riding similar elevation for the day.
I follow a different jetting guide that has worked perfectly to really dial in the jetting of all the bikes I've owned or presently own. Here is the procedure for the idle (pilot jet) circuit and how to determine if you have the correct pilot jet...
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...start with the pilot circuit. Turn the air screw 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 air screw 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 air screw for the best response.
Now is the time to determine if you have the correct pilot installed in your carb. The air screw position determines this for you, making it very simple. If your air screw 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 air screw for the fastest idle, it's time to tune the air screw 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 air screw 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 air screw is not a set-it-and-leave-it adjustment. You have to constantly re-adjust the air screw to compensate for changing outdoor temps and humidity. An air screw setting that is perfect in the cool morning air will likely be too rich in the heat of the mid-day.
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