I picked up a stock YZ450F exhaust (just the exhaust, still running WR header). After I installed it, I have a loud "pop", "back fire" or whatever you would like to call it, when I let off the throttle. Just one, but it sounds like a fire cracker. I've done all the free mods Here are the carb settings... Current jetting 165 main, JD Red #5, 48 Pilot, 1.25 turns, 68 starter jet, #50 leak jet. I took the mid-pipe gasket from the WR stock exhaust and put in on the YZ (the YZ looked a little tired). I also bought a new header gasket. I've tried a #50 pilot. I've tried a 168 main. I've tried backing the fuel screw out. Like I said, it only does it once, but it is so loud it's almost embarrassing. I seen where some "popping" is normal. I suppose I can live with this, but I would like to tune it down. Last resort will be to put the stocker back on (which I've done, and the popping goes away - although I think it's still doing it, but it's just "muffled" now). Thanks in advance and Happy Easter to all.