what are the best jet's for a 400exand how much power will this add

 

Jets alone wont do much, if anything....you gotta get the bike to breath (ie. better exhaust, and intake stuff) in order to get use out of your jets. The stock jets are set up for a stock motor and intake/exhaust.
Ryan

 

You can get a dynojet kit, it's supposed to help but I think that it's hard to get more power from a stock quad with just jets