The air cleaner's an issue whenever it's dirty. If it's blocking substancial airflow, it's going to cause the bike to run badly.
I clean mine every 3-4000 miles, or sooner, depending on what type of riding I've been doing.
ALWAYS start with the simple stuff. Pull the filter out and clean it and re-oil it. Install and see if it takes care of the issue. If not, go to the next cheap and simple thing and replace the spark plug, but just do one thing at a time, if you can.. If you do a bunch of repairs/modifications at the same time, you won't know what actually fixed it.
And if this doesn't solve it, I'd look into the low speed air jet screw on the bottom side of the carb. There's an aluminum cap over it, which has to be removed, and then lightly bottom it and back it out 2 1/2 to 3 1/2 turns. My bike felt like it was surging or stumbling at regular highway speeds, on flat ground. I checked my screw and it was fully seated. I backed it out 3 1/2 turns and it's been fine since.. That was 4 years ago.
Last edited by Paper; 09-25-2009 at 09:04 AM.