It is common for these little engines to pop like that on decelleration. When you let off of the gas your basically engine braking until the rpm drops low enough for the clutch to disengage. Similar to how a big rig uses a Jake Brake to slow the rig down, except your not restricting the exhaust side of the engine. It really depends on how loud the popping is. If it's sounding like a huge explosion then more then likey it's lean pop...
What you really need to do is make a high speed run on your bike & then kill the engine while the engine is still at high RPM. After that you should pull your spark plug out & check the color. The lighter the color the leaner the engine is running. The absolute best way to run this test would be to get the engine warm by riding it around alittle. Then bring it back into the garage & install a brand new spark plug. Then do the the test & check your plug reading.
White / Light Grey = Too Lean
Tanish Brown = Just about perfect.
Black = Too Rich
Here is a link to better explain this process.
Reading Spark Plugs Correctly