2005 Yamaha bruin 250 won’t run with choke off

I have a 2005 Yamaha bruin 250 it will not run with the choke off and when it does run when u get past half throttle it spits and sputters them wants to die and will not restart unless the choke is on I have taken the carb apart numerous times and cleaned it I have looked the whole carb over for a air/fuel mixture screw and can not find one any where can someone please help thanks