Been burning about a month now off and on. Mostly 2 yr old red oak thats been covered with good air flow for 2 yrs now. Burned a little poplar that was dropped last yr and I split a few months ago. It felt dry as hell. Could the poplar be the culprit? Liner is unlined btw in an external chimney. Whata yall think? Looks like too much to me. I always get a good hot 1 hr burn fire before I reduce air yet it still smokes a bit. I was thinking that since I have about a 1/2 brick worth of slack in my upper firebrick which I split on the sides, that I could seperate them in the center causing a better draft and a hotter flue temp which could reduce the build up.