I agree with Gambler,, try reducing the blower setting back to near where it was.. it doesn't take much of an increase in the opening to the fan to increase the airflow through the fan quite a bit.. try the original setting first, then increase the opening only an 1/8" at a time. Unless you have an increase in unburnt coal in the ash you don't need more air.
I'd try this adjustment first, it is one of the things you changed that brought about the smell, so I'd change it back.. One change at a time, wait for a result..
The Hopper/stoker do not need to be sealed,, like mentioned above, they are under a slight vacuum.. IF the chimney is drawing correctly..
There are 'draft inducer' s availble,, do a Ebay search or a search on this site, there are a few members that use them.
The dimensions of the chimney you posted, are these inside diameters?? if so they are pretty large.. I would think that 8" would be plenty,, unless this is a much larger unit than I'm visualizing.
Even in a warehouse you should have a CO detector.
And PLEASE !! turn off your caps lock.. typing in all capitals is considered YELLING !! Thanks..
Hope this helps.. Greg L