Welcome to the forum!
Not likely valves...
Do you have any modifications to your airbox? The main jets control somewhere between 1/2 to 3/4 through wide open throttle, the needles from `1/4 to 3/4 throttle and the pilot jets/fuel screws from idle to 1/4 throttle or so - So it sounds to me as if your issue is either neeldes or mains (or both)... mods to the airbox without rejetting with richer mains and changing the needles to aftermarket, more agressive taper, can cause symptoms just like yours.
'01 660 Raptor, Burgard +2+1 arms, YFZ Shocks, LSR anti-vibe, CTR pipe, jetted, K&N
'01 Polaris Scrambler 400, K&N
'02 Honda 400ex, Big Gun pipe, jetted, K&N, Lonestar +2 A-arms, Lonestar anti-vibe
'04 Suzuki LTZ400
'16 RZR XP4 1000 Turbo
Last edited by QuadManiac; 10-25-2015 at 08:25 PM.