I have a white bros e series slip on and a uni filter. I use an outwear cover on the airbox and run a 155 main and 40 pilot. Befor the rejet I got a lot of let off backfire/popping. The quad ran like a bear for a couple of days until yesterday. It started out running good with little to almost no popping when I let off. Then after riding it like a madman for a couple of hours straight it started popping like crazy again when I let off the throttle just like I never rejetted and it was running lean. Any insight?

 

If you're running with no airbox lid and are anywhere near sea-level, you're probably running real lean. You probably need to be somewhere closer to a 165 main. Try putting your airbox lid back on. If it runs better, then you need to up the jetting.