Listen to Clark, he is the Undisputed Wisconsin WSM Guru. At this time with the outside temp at 17 degrees my 18,5 wsm is cruising at 240 with all bottom vents about 1/4 open and the exhaust fully open. My question is did you have the top off for a while doing something or did the temp just spike? If you have the top off too long it floods the basket with air and it takes off, doesnt take long for this to occur.
Backwoods Fatboy,2 Primo XL's, 2 Ugly Drums, 2 22.5 wsm's, and 2 18.5 wsm's.