I'm basing this on experience with the smaller WSM, but I don' think the big boy would be that difference. With even a tiny gap at the bottom vents and the top closed your fire should have starved or at least died down. It sounds like you have another source of air getting in. How is the fit between the middle section and the bowl? With the smaller WSMs the middle section has been out of round when received. Also, check the fit on the access door to see if air could be getting in there.
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