For example when you adjust the idle jet (1st. thing) you will start the bike and run it for a few seconds and then shut it off. You will never touch the throttle because that would allow the carburetor to function through the slide needle and main jets thus fowling a true idle jet only plug reading.
For a slide needle reading you would not open the throttle farther than the stage of the needle you want to test. Also carburetor jetting is cumulative so if the idle and slide needle are off it will give you a false main jet reading.
Regarding main jet plug chops you will need a load on the engine to keep it from going pop. The only thing that matters is that the throttle slides are wide open. The actual load on the engine is doesn’t matter (gear, speed, whatever). Also be sure to turn off the engine before you let go of the throttle, and 10 sec. of WOT (wide open throttle) is a long time! You should only need 3 - 5sec. or so to get a good reading (works for me).
All of this is assuming you know how to read plugs, and you have the correct carburetor size, needle slide, slide needle, and slide cutaway; because all of those things will effect jet selection. Also altitude and barometric pressure will also effect jetting so when in doubt jet rich! Wiping splooge from your cans is cheaper than rings and cylinder plating.
When I do plug chops I jet it rich then ride with someone around town. I see what conditions cause hesitation and have a friend tell me when I am spewing smoke. Then, I will "fine tune” the jetting again. When properly done plug chops won't give you the exact jetting but they will get you close.
I hope this information helps.
