Both of my SBC tail pipes lay black patches on the shop floor for the first several minutes after cold start up at idle. One has an Eddy 1405 and the other is a Rochester 2bbl. I haven't touched the idle mixture screws since I've owned either of them but plan to in the next few days to try to improve on this. As I'm overthinking here, I wondered if valve seats or valve seals could ever get so worn that they produce symptoms that mimic a "rich carb condition" out the exhaust? I would tend to think if that were the case you'd have poor compression in the cylinder with the worn valve(s) at the very least? Please correct my thinking as you see necessary.