I agree with RonL.
Also, when firing up a WSM you want to catch the temp before it gets to your target. If I plan on cooking at 250, start closing vents at 200.
Another thing, I have a heckuva time ever getting my WSMs much over 300. I have to really want a hot WSM. In other words, mine don't get there without help. How accurate is the stock thermometer? Did you verify temps with a different thermo?
Just some things to consider.
My Boys: Joseph (EI); Andrew (EE); Daniel (EE)
My Toys: 10 WSMs (B, C, DH, DA, EZ, DZ, EO, DU, DR, EE/DU mutt); Brinkman Cimmaron; Chuckwagon Cooker; 4 22.5" Bar-B-Q-Kettles; 2 18.5" Bar-B-Q-Kettle; 1 Red OTS (H); 3 WGAs (EZ, DE, N); 3 SJPs (DT, DR, DR); 2 Smokey Joes (A, K); Genesis Silver C (DU).
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