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What size WSM (18" or 22.5")? I did a 22# T-bird whole on the 22.5 kettle this past yr. It was moist and tender! I've not done one on the WSM yet. The WSM manual says you can do from 8 - 18 # birds. The virtual Weber Bullet says "You can cook most any size turkey you like in the WSM as long as you use a vertical roasting stand on the bottom cooking grate. It's common for people to cook 22 pound turkeys this way. The biggest turkey I've ever seen in the 18-1/2" WSM was a 28.5 pound bird cooked vertically on a large pineapple juice can." Here's a link to a photo of a 23#bird: http://imageshack.us/photo/my-images/199/img0703md.jpg/
Its the big boy 22 1/2. Only turks they had at that price was like 23+. Im not even gonna try that large of a bird. I opted for a 12.5 breast bone in thawing as I speak, Then brine her up and smoker that stuff.
I have do 20 to 23 lb. birds on my WSM most every Thanksgiving, I start with my temps up high between 325 and 350, then after the first 4 hrs. I damp it down to 250 for the rest of the smoke.
Last year for Thanksgiving, I did a 26# turkey in my WSM 22.5". I brined it overnight and started the meat cooking on the smoker at 7am thinking it would take 5 hours. At 930am, the bird was completely done! I reduced the temp of the smoker to about 160 and left it on for another 1.5 hours. I guess brining really reduces the cook time. It was the best turkey I have ever had. We have been frying our turkey for years because of how juicy it turns out, but the smoked one was even juicier!
Here's a pic of the bird next to the fried one we did. Smoked on left, fried on right.