Bogging in 3rd and 4th

Hopped on the fourwheeler for a ride the other day, while in 3rd and 4th it started bogging down. I disconnected my snorkel and put the factory snorkel back on and it ran fine other than it was backfiring when i let off the throttle in high rpm range. Checked my plug and it looked rich. I'm running a 170 jet and my air/fuel screw is turned out 2.5 turns. Should I try turning in the air/fuel screw? If so how much should I turn it in at a time, I didn't want to try this at first because I knew all of you guys are running 2.5 turns out. Thanks.