First ting that comes to mind is you intake air is lacking or your stack is to skinny. Offsets are all about the flow of air the faster the air moves through the pit the better( more efficient) they run. If the intake is too small you will have a hard time regulating temp or achieving a higher temp. If your Stack is undersized to skinny you will have a hard time getting a clean fire because the smoke backs up into the fire box and displaces the clean air the fire dies, to short and it wont draft ( pull fresh air to the fire).
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