loosend,
You can rejet with the allen screws in/carbs on, just be vewy vewy carefull of the floats and the float needle and the mounting pins. The floats fall down when the bowls are removed and sometimes the pins and the needle go places you don't want them to...
Make sure everything is in it's place before you put the bowls back on. This is especially true if you do this while the bike is on the kickstand, at an angle....
