If it is all tight, I would suggest checking your flue height it may want to be higher, 31" sounds very short. I am assuming that your are referring to the riser when you say flue? If you raise that higher that
should add to suction. As well, what is your gap between the riser and the barrel top? it should be around 2" give or take 1/4".....some things to check....
hope it helps.
PS. I am not an expert, just a guy that has messed about some
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