When I was a kid I remember my great uncle Raz telling me that if you aint in charge of the airflow then you just aint in charge. He was referring to dark fireing tobacco in a tobacco barn but the principal is the same. Do what you can to seal it all up so that air is forced to go where you want it to - through the cook chamber and up the stack.
Was 150 all you could get out of it?
How long did you wait for temp to build?
What is the total square inches of baffle gap at the far end?
If you've got a couple hours try this:
Open your stack 100%
Open your draft to 8 square inches
Light 2 charcoal chimneys at the same time
When they're all white pour them into the center of the firebox grate and push them into a pile.
Pour one more chimney of fresh unlit briquettes gently on top of them so they are all touching.
Close it up and start a clock set to straight up noon.
Record the time/temp every 10 minutes for 2 hours and don't adjust anything.
Once the temp stays about the same for 2 readings - close the draft to half and note it.
Post your times/temps back here with a picture of the draft.
Or you could call Frank and have him calc it and solve the mystery real quick ...
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