yep do what Nelson said - these bikes do pop a lot and those pipes are known for it too. You might get rid of it with some fine tuning but you might not. Also make sure they're bolted down right because an exhaust leak would cause popping.
Every V-Twin (harley Type) I've ever had would pop (blue flame at night) on decel with free flowing louder pipes, I for one have always liked it, lets those around you know what your riding!
One more thing is: Are the pipes open or do you still have a baffle in them? Lean fuel mix and open pipes will cause it to pop. Also have you added a cam as that may halp to increase the level of pop.