02 sportsman 500 ho…. has run great for 7 years. last year it started to sputter and bogg down and cut out but only when going up steep hills all other times runs perfect..so changed sparked plug and fuel pump and took it out and it does the same thing..so this year replaced belt and air filter and took it out and it still does the same thing so I removed the airbox cover and went up the same steep hill and the machine has so much more power with the airbox cover off and does not cut out at all this is how it use to run before the problem started…Im lost on what the problem is because it ran fine for years and just started this out of the blue..baffled why it runs so good without the airbox cover on..could there be a fuel flow issue and the added air without the cover on increases the fuel flow???..i want to be clear I have not change modified anything before this problem started >>>Please help