This was during my first use this weekend, so I'm sure I did something 
I decided to try a chicken for my first smoke on my WSM 18.5". Everything set up very easily and seemed to go great. I used the minion method and placed about 3/4 of a stack in the center. The only issue I remember is things started to cool off after 2.5 hrs. I opened the door and gently moved the side coals towards the center. That took care of it in short order. I had the temp dialed in to about 225 on the iGrill 2 (ambient probe) and it stayed pretty constant for about 4 hrs (minus the small dip which was easy to fix). For some reason, at the end of the cook when the chicken was at about 158 degrees (so close) the temp spiked over 35 degrees in 6 minutes (per the iGrill graph)! I must have missed the alarm on my iPad, but when I looked it was now at about 260! I immediately closed two of the lower vents and left a crack in one. The temp started to fall, but it was very slow (3 degrees in 20 minutes). Not that big of a deal since everything was almost done and I'm sure it helped speed things up a bit However I have no idea how the temp could have soared that much in such a short time. It wasn't windy and the WSM wasn't sitting in the sun. In fact, I only had to go outside to adjust the small dip. It was essentially just chugging along then BAM! Any ideas?
I decided to try a chicken for my first smoke on my WSM 18.5". Everything set up very easily and seemed to go great. I used the minion method and placed about 3/4 of a stack in the center. The only issue I remember is things started to cool off after 2.5 hrs. I opened the door and gently moved the side coals towards the center. That took care of it in short order. I had the temp dialed in to about 225 on the iGrill 2 (ambient probe) and it stayed pretty constant for about 4 hrs (minus the small dip which was easy to fix). For some reason, at the end of the cook when the chicken was at about 158 degrees (so close) the temp spiked over 35 degrees in 6 minutes (per the iGrill graph)! I must have missed the alarm on my iPad, but when I looked it was now at about 260! I immediately closed two of the lower vents and left a crack in one. The temp started to fall, but it was very slow (3 degrees in 20 minutes). Not that big of a deal since everything was almost done and I'm sure it helped speed things up a bit
However I have no idea how the temp could have soared that much in such a short time. It wasn't windy and the WSM wasn't sitting in the sun. In fact, I only had to go outside to adjust the small dip. It was essentially just chugging along then BAM! Any ideas?