HT beat me to it- it does look like the burnpot liner isn't sitting down all the way- when it sits in the burnpot air comes in through the big pipe on the right and up through the liner holes. Could it be yours has distorted or something in the burnpot itself to keep air from dispersing into the burnpot liner as its supposed to? The convection blower flap is only for the uneven warm air being delivered into the room, not this burning issue... did you say you had a break in the droptube area? Could that affect air flow anyway coming into the burnpot and liner??
There was no break in the tube, there's a pic in a prior post. The welding had chipped off the top of the tube, I've since patched it with cement at Enviros request but I still get the same issue. I may mess around with the burnpot a bit because there may be something distorted?
This is for sure an airflow problem but I (we) can't figure out what's causing it.