Help continued

[Patience]

I would first try and secure that mounting plate tight to the wall.  The collar I think is suppose to go on the inside of the box, and kind of pull mounting plate tight against the outside wall.  It looks like you may have to extend the slots in the collar a little bit in order for it to fit.  In another post from ranman, he posted the Bradley tech saying the collar dose not have be used as long as the mounting plate is secure to the smoke box using longer screws etc.  Some high temp silicone to seal it up wouldn't hurt.

The second thing I would try is to just exit the box without the elbow or extension.  This will create more of a direct vent out.  I had a fishing shanty stove once that would back up smoke because the extra elbows and extensions would heat up enough and I would lose my air flow to the path of least resistance as Gizmo said, and it would puff out the holes of the damper handle!

I piped my 4inch exit out the side near the top for convenience sake, and the only intake is thru the Smoke generator itself.  This helps create a pretty direct path from the generator to any exit.  The only time I get smoke out the generator is if the damper is closed too much which makes sense.  Could it be there is more air coming from another inlet source?  I think if the only intake is thru the smoke generator hole, this would be enough.  With gas though, I don't know if you can seal off all other inlet sources.

Hope you get it figured out.