Hey,
Funny you mentioned it. I used to ride an 87 XR200 (no pumper) with XR's Only pipe and it would pop like that on deceleration all the time. I never thought much of it, just thought it was the pipe. This summer, I bought a 96 XR400 (no pumper, yet) and also put on an XR's Only pipe. It does not make the pop at deceleration. What's the difference? It's a fact that my 200's valves need adjustment, and even though I tried to adjust my chain, it still rattles. My 400 was serviced right before I bought it, so the valves are adjusted good, and the chain doesn't sound loose. I believe DualSportr is correct when he said it could be cause by timing chain, ignition, and in my opionion, valves out of adjustment. Looks like you should check your timing chain and valves.
So what's you take on that Pumper carb, Biker? I've been strongly considering one for my bike when I get the extra money. Good luck.