I'm thinking that temperature is temperature and it doesn't really matter what you are cooking it on, if its 225 on a WSM, and 225 on the BWS (that you traded and I will end up with someday), or 225 on the Lang, I cant see where its the cooker that causes the plateau, most likely it is the piece of meat, like Beerguy said.
Honestly, I cant see where the plateau's are caused by the WSM, you must learn to adapt to your piece of cooking equipment. Maybe we can get together some day between 6am and 11pm and I will show you how to cook on a WSM.
Rubbed, Smoked & Sauced BBQ Team
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