Hey Rocketjock:
1st of all, did you remove your carbs and drill out the blocks for the a/f mixture screws?
This in my mind is a must if you installed a new pipe as the stock setting of the a/f mixture screws is lean. This will help more than the shims for decell backfiring.
The shims that chupacabra sent me look exactly like motherboard washers. Radio shack once sold smaller & thinner washers, but were discontinued to my understanding a while ago to the present ones. I did install these on my B-12 and they work fine. As long as they fit in the hole, they will work.
Keep in mind that the shims will only affect mid range operation of your carbs. Ideally you would want to also drill out the plugs on your a/f screws which helps adjust idle.
Carb mods to make things simple work in steps. The a/f screw adjustment helps low rpm fuel supply, the shims help mid range fuel supply & changing the main jets helps things over all. A perfectly tuned Bandit really requires all 3 to obtain a perfect setup, along with air box mods.
The poping you hear on deceleration is normal, and while carb mods helps this, it will always be there to some extent.
If you are happy with the way it's running, and just have some minor decel pop, don't be too concerned as long as you are not running too lean.