You can look in the carb inlet while you adjust the idle screw and see if the slide moves up and down. Sometimes the idle screw wears a groove in the slide and won't push it up as far.
Sounds like the engine is running a bit rich. The appearance of the plug will depend on when the chop was done. If you turn off the engine at open throttle, the plug may look different than chopping at idle.
You may be able to get by with a higher fuel/oil ratio with 2-cycle oil compared with ordinary 30W. The plug may look wetter with 30:1 30W. Some excess oil is safer than too little, though.