I have had my YS640 for about six months now. I absolutely love it. I use it 2 - 3 times per week. However, the last few weeks I have noticed a black liquid dripping on to the meat. It appears to be coming from the smoke stack, though I'm not 100% sure about that. Has anybody else seen this?
The smoke stack is supposed to be level with the inside wall. If installed correctly there should be no way anything can drip off the stack onto the food.
Are you using any 3rd party smoke generators in the cooker? If so, these can create a tar creosote that will accumulate on all surfaces, including food, when can drip, which appears to be black spots. Read this article, as it has a picture of this from excessive use of this type of smoke generator: viewtopic.php?f=36&t=1365
Also, do you clean the inside of the lid and the upper grate, as cooking residue can drip from these as well?