Usually taking off the air filter makes it run a bit more lean because it can get more air. So I'd assume if it runs right without it, putting it on will make it run more rich.
Try making the carb leaner and put the filter back on. As always, when playing with different carb/adjustment check your plug to make sure you aren't running to lean.
Try making the carb leaner and put the filter back on. As always, when playing with different carb/adjustment check your plug to make sure you aren't running to lean.