I have done a couple of Butts on our 22.5" OTG. Did not use a roto. Both were in the 7-8lb range. First took about 6.5 hrs, second about 7.5 hrs. Depending on how you arrange the coals you will have to tend it every 1-2 hrs to keep the fire up. I used RO Lump so if you went with Stubbs Bric's or similar you would get a longer time between tending. Kept the temps in the 250-275 range. Rotated the Butt about every 2 hours. Time is not what you watch, watch the temps on the butts, the old saying goes...."its done when its done". Also, the "loose bone test" works very well to tell how ready she it for "resting" Once off the heat our were wrapped in foil, covered with towels and let to rest for 2 hours on the counter.
Ours turned out GREAT, added wood chips for the first 4 hours of the cook time each time I added more lump. When I would add fuel, I would not "remove" the lid all the way, just slide it to the side to get access to the coals, this way I retained more heat and it has less of an impact on the cook time.
Used a store bought Rib Rub, did not mop. Rubbed it and wrapped in cling wrap overnight in the fridge. Let it out of the fridge for about an hour on the counter and to the grill it went.
Good luck and let us know how it comes out!
Terry
