good news on the fix, glad you got it figured out. how did you cut back the air intake on the combustion fan, tape as you mentioned earlier? i know others have used flat refrigerator magnets on the fan cage to restrict airflow or they came up with a home engineered metal baffle plate that mimics what is on the harman combustion fans.
this is important info for new stoker users to be able to search on....
if you have more combustion air forced into the firebox via the fans(s) than the exhaust can get rid of through its draft, then it has to come out somewhere whether it be the hopper, faulty stoker gasket, window air wash vent, etc.
indications this is happening - like you experienced is the co reading on a monitor &/or rotten egg sulfur smell or moisture on the inside of the hopper lid.