I got a stock 05 450r - when i take the spark arrestor out will i have to change the jetting? maybe put a 120 mian jet in?

 

yea you will have to change the jetting around try 1 or 2 sizes up from the stock size

 

Just in case you didn't know (not impeding your intelligence), you are REQUIRED to have a spark arrestor to ride on Forest Service land.
But yes, as bombrider04 said, you will have to change the jetting, not by much though.