Root Diameter - Needles are offered in several different root diameters. The jet needle sits in a hole in the needle jet, as mentioned. The clip position determines how deep it sits in the hole. The root diameter, on the otherhand, is the diameter of the needle at it's pointy end. The wider the root diameter, the smaller the space between the needle and the hole in the needle jet. Therefore, I needle with a larger root diameter will be leaner than a needle with a smaller root diameter. The root diameter overlaps with the slide cutaway, which is to say that it affects primarily 1/8th to 1/4 throttle mixture. Typically you would swap for a needle with a larger root diameter to compensate for high altitude (or extreme heat).
I'm trying to understand how the needle affects the pilot circuit at 1/8 throttle, or just above idle. I thought the needle only affected 1/4-3/4 throttle openings, but in practice that is not true. My BEL is not as rich off idle as the CEK. So the diameter (and maybe the taper??) affects the pilot circuit - but how? Is the explanation above correct? Does the pilot circuit cross the needle at some point?
The article above says that the root diameter is the diameter at the pointy end? Is that true? That is different than the Sudco drawing, which measures the diameter at the clip end (
http://www.sudco.com/vol33/135-136.pdf) This is info. that I have seen in more than one place. But how could the "clip end" diameter affect the pilot circuit/idle??
As you can tell, there are a couple of things that I do not understand!
Jeff
BTW - The info. you guys have given above is excellent. Thank you.
There is a picture of the inner working of the carb - and more great jetting advice - here:
http://justkdx.dirtrider.net (tech tips/carb tuning.)