Originally Posted by Harbormaster
Sounds to me like your center section is out of round.
Because there is not a rolled lip on the bottom of the center section, it can get squished and it won't fit the charcoal bowl right. Air gets in and makes it difficult to control temps.
Check with a tape or yardstick across the diameter. It should be a consistent measurement.
I am willing to go out on a limb and tell you with near 100% certainty this is your issue. Reason I say that is I had fought the same issue with mine since I first bought it in 2005. Over the years I have had my WSM, I have run it with every possible configuration of water pan, clay saucer or foiled water pan possible with the same basic results of eventually high temps. It wasn't until I finally got fed up with it and bought a set of Nomex gaskets
for it that I finally got it to seal properly. As part of installing the gaskets I totally cleaned my smoker with oven cleaner. That was the first time that I had it cleaned totally since 2005. When I did that I found that I had a dent along the side of where the door mounted and that the bottom base was out of round. All these appeared to have slight gaps until I did the deep cleaning and then it showed they were major areas of leakage.
I got it all back to round and then installed the gasket set using RTV silicone. Here are pictures of the gasket set installed.
I have only done this overhaul fairly recently so I can't say how well the Nomex will hold up over time. But I can tell you that since I did this I now have positive control of temps like I never had them before for both short and long cooks. To be honest, I think the gaskets are the icing on the cake, but finally getting the middle section properly sealed was the key.