Hi William, now that the season is over I may have some help for you, all be it a little late. I have a little experience with the 206 and can tell you a few things that may help.
You can read the plug easily just use a new plug to get accurate info. The boot does matter and is definitely an important part. Yes it can produce an air leak. The low speed jet is so small make sure it is clean and clear. A sonic cleaner works very good with distilled water and a mild water based cleaner. The needle is for low to mid and does not “cross over” to the high speed system. Although the needle can be an indicator as to the direction you may want to go on the high speed. Now for the high speed – remove the carb and remove the bowl. With the carb upside down measure from the lip of the body going around the float to the top of the float. Using a caliper set it to .87 inch. Make sure both sides are measuring the same and are not uneven. High speed can be adjusted by the float setting. Yes it works. Short tracks and rich setting at about .83 to .84 and lean setting to .92 to .93 – keep in mind the carb is upside down. Basically you are starving the engine slightly with a low (right side up) float setting. The engine loves to be lean on the top end, but if course there is a limit. Short straights and short twisty tracks on the richer side maybe like Davis. Longer straights and faster track leaner like Sonoma or maybe Atwater( Atwater being a compromise between the long straight and the slow tight corners) In conjunction with precise valve settings a fair amount can be gained. Also make sure the air needle adjustment is about 1-1/2 turns to start out with. Remember that it lets air in, the further out the more air is entering the low speed system. The needle in position 3 – numbered from the top of the needle with the top being #1 so #3 is in the middle. Experiment and learn. You may private message me and I can help a little more or clarify what I write. By the way, I have never had an idle issue in the 3-4 years my son and I have raced the 206.