if you look at your airbox there should be two inserts one holds your sponge filter in place the other smaller tube actually restricts the airflow to your carb so remove the smaller tube but keep the filter tube in there to keep crap out your carb.

yuo will need to adjust your idle valve and adjust the screw at the bottem of the mikuni carb just remember you will need to test it every adjustment to get it right and only turn the screws a quarter of a turn for each adjustment