Make sure that there are no exhaust leaks, anywhere. On decel any fresh air that can be drawn into the exhaust system will ignite with hot unburned fuel and explode, causing popping and backfire.
I'm going to cast my vote with Joe.
The leaks probably are in the first 18 inches of exhaust tract.
Make a little smoke by feeding it a little oil/gas mix or diesel through the carb and watch for leaks near the manifold joints.
Plug the exhaust outlet with it running while a helper watches and listens around the manifold.
EXHAUST LEAK
