I just built an UDS and tested it out on a whole chicken and some ribs. After a couple hours I noticed this black slime on all the food. I think it is coming from too much moisture from the lid. I have two exhaust tips on the lid. I read about "dirty fire" and there is white smoke coming from the exhaust tips. What I don't understand is that if people soak their wood anyway how does it being wet make it a "dirty fire"? How do I fix this. Thanks.