Hi guys, I've just spent the last few days trying to get rid of the popping on decel from a 05 WR450F. And I can't believe how simple the fix was so I want to share it for anyone else suffering like I was.
Basically, the first thing I would do before you go stuffing around with the jetting, or hunting and plugging air leaks in the exhaust is just to check your ACV. Mine must not have been seated properly - even though it looked like it was when I pulled it apart. If you don't know what it is:
Just pull the cover off (watch for the spring) and check the condition of the rubber for leaks and then re-install and see how it goes. For me, it fixed it straight away. Make sure the pointy bit goes in towards the carb, if you were wondering.
I pulled the cover off accidentally while man-handling the carb while re-jetting following an exhaust replacement. When I pulled it off again it looked fine but after reinstalling it - the problem was completely gone.