If a bike requires more air to run properly, then that means it’s getting too much fuel. That’s why when you take the air box off it runs better. I would pull the carb bowl off and check the number scribed on the main jet, and then get a jet one number smaller. Put the air box back on and see if that fixes it. Also, you may want to do a plug chop. That will tell you whether you’re running too rich, too lean or just right. You want a nice brown color. Search the wiki for “plug chop” to see how to properly do it.