Well, the first glaring question I have is are you starting with hot water in the pan? It takes a lot of BTUs to get cold water to boil.
Have you tried w/o water? Wad up some foil into balls, put them in the water pan, cover with a sheet of foil.
Also keep in mind that when you get the WSM lit, it will take a while to heat up. Give it a while, watch your temps, when you get to say 220, then shut down your intake vents to 1/3. Always leave the top vent wide open. When you put cold meat on, you can guess what's gonna happen to temps too.
Sounds like you have the stock Weber thermo too. I don't have any of those so I can't speak to the accuracy.
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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