Which jet circuit in the carburetor is effective depends on the throttle opening. Your main jet is effective primarily in the range from 3/4 to full throttle. The needle is the big one - it affects the mixture from 1/4 to 3/4 throttle. So if you're doing a plug reading, you want to run at 1/2 throttle for a minute or so, pulling upwind or up a hill, say, and then suddenly hit the neutral finder, if so equipped, or pull in the clutch, and turn of the key, coasting to a stop at the side of the road. Where you take the plug wrench out of your pocket, and pull out the plug and have a look. White plug, clip down, black plug, clip up.
For Main Jet tuning, repeat as above, but at full throttle. White plug, bigger jet, black plug, smaller jet. If it won't get up onto full throttle because of bucking and missing, you have to look at your pipe and if you see black smoke, you know it's 'way too big, so you'll have to try a smaller one. If it's too small, you'll get a white plug tip on full throttle, if not a lot of starving, bucking, missing, spitting back from the carb, (classic weak mixture) etc. Your main jet will have to be an awful lot too big before its size will affect the mid-range of the needle adjustment.
The needle position should already be set by this time. In extreme cases, you may not have been able to get a rich enough needle setting, because your mainjet may have been too small.
Variations in main jet size shouldn't affect needle setting, although if I've made big main jet changes, I always check the needle setting again.
The first 1/8 of throttle opening is affected by the pilot, or idle jet setting, and throttle cutaway affects the "transition" from idle circuit to needle. Adjust pilot jet at idle, alternating between the idle stop screw to get the lowest possible idle, and the pilot jet screw, to bring the idle up as much as you can. Throttle cutaway can only be changed by changing your slide, but fortunately, this is an adjustment that is seldom necessary. The indication that you need to will be hesitation (or worse) as you increase throttle from just above 1/8 to the needle range.
That's about all there is to getting your carb tuning "in the ballpark." Do remember that too lean a mixture will bring about overheating, often detonation and subsequent holing of the piston(s).
