I agree with Steve R. 's comment that the major contributor may be that wide open vent. Read this section of Max Good 's review of the PBC which shows what happens when the vent is set wider open than optimal for the elevation:
Our elevation is about 800 feet so we adjusted the vent 1/4 open. Later, we tried setting it at 1/2. The PBC got real hot for a couple hours then, even with plenty of coal remaining, the temperature dropped like a rock and almost died out. When we returned the damper to 1/4 way open, the fire came back and temperature began to rise.
So, as I mentioned, I've been pretty intrigued by this. I'm at about 75 feet ASL and this weekend I cooked a tri tip for an hour with the bottom vent 1/4 open. After, I set it at 1/2. The temperature went up to 375 for 2 hours, then dropped to 300. At that point I closed the bottom vent to 1/4, but the temperature never rose and continued to drop.
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