One of your better sources of information for high altitude riding with a 660 will come from @Ridgway81432. I seem to recall reading posts from him that he doesn't change the jets on his 660 but more so, drilled holes in the top lid of his airbox to help increase the flow of air... he can confirm on his recommendations for high altitude riding on a 660. Certainly one way to address being down on power is to magnify your low end torque by way of CVT mods and Ridgway is well versed in that as well. I've never had a 660 but CVT mods are how I've addressed being down on power here in CO on my 700. 700 being fuel injected.
One other item to point out that many don't think about... make sure and check/adjust your tire pressure when you arrive. It will have increased by quite a bit. But conversely and probably more importantly, make sure you are aware that your tire pressure will decline as you make your way home where you could end up with very low psi in the tires. If securing your ATV will be affected by tire psi lowering, just make sure and check tire psi, add air during your travels when needed. Not saying you need to stop every hour to check but maybe check on your first/second fuel up during your return trip.
One other item to point out that many don't think about... make sure and check/adjust your tire pressure when you arrive. It will have increased by quite a bit. But conversely and probably more importantly, make sure you are aware that your tire pressure will decline as you make your way home where you could end up with very low psi in the tires. If securing your ATV will be affected by tire psi lowering, just make sure and check tire psi, add air during your travels when needed. Not saying you need to stop every hour to check but maybe check on your first/second fuel up during your return trip.
