I found this thread from a brethern. Sorry to say I dont know how to transfer a quote from another thread but he had the same problem and this is what he found out
"Thanks for the reply. I may have just answered my own question. I'm five hours into it since I lit it for a dry run, four hours after closing the dampers and its almost to 325. I put a firebrick over the exhaust to try to choke it out more in the event the damper had any gaps (intake is already blocked by guru plate and plug). When I did this, I could see smoke blowing out under light pressure. It getting dark and having a flashlight actually helped me out. I decided to try to do a reverse smoke test. A smoldering piece of wood and a flashlight gave me my answer. After checking all the door seals and the damper, it looks like the culprit is the closed damper. It was sucking in the smoke like nobody's business. I don't really see any gaps in it, but the draft is so good that its sucking it through somehow. I guess that amount of draft with the heat produced with lump is enough to keep her cranking. It's just a little strange; I'm used to closed dampers = no fire."