For what it's worth I live in Colorado (alt 6500) and have ridden in the high country with my 2004 660. I changed the main from stock (#145) to a #140 main jet. I didn't have to make any other changes and she ran great. No matter how you tune it, it's gonna lose power at 10-13000ft. But I absolutely did not have any problem with it blubbering at altitude. With the same set-up I was able to do Moab Utah without any problem (although it was getting a little lean). Had I went any lower I would have changed back to the stock jetting. Will it run on stock jetting, sure it will just not sure how well. Mine ran like a raped ape without doing any air box drilling for altitude. My 2 cents.