The little screw is your air mixture screw. It adjusts throttle response at idle & low RPM. Don't mess with it for rough tuning. Standard is all the way in and then 720 degrees out.
A plug chop is done after running a mile or so with a warm engine on wide open throttle (WOT). You kill the engine keeping WOT.
Upsizing till you 4-stroke, and then going down till you don't is a good method to tune in a carb. Just make sure to do a plug-chop after you've downsized to make sure you're not running lean.
Good luck!