I've always been told, if you are trying to tune the carb by reading plugs you have to pull them immediately after making a pass (i.e. Put in a new plug, make a full pass, shut off motor, coast up to the time slip booth, grab your ticket, pull to the side and pull the plug & read...otherwise you are going to get mixed readings with all the idle/part throttle stuff going back to the pits).
I hope that helps ya, happy tuning.