I'm using #88 with a 72cc with stock cylinder head, airbox and carb.
By the way : with a 4T it will always take a while before the plug picks up some colour at the tip, that's why you need to do a "plug chop" if you really want to be sure quickly, the colouring starts at the bottom of the insulator, which you can only see if you literally remove the threads of the plug...