This has happened on the last two carbs I've done. The pot metal has gotten a black coating of something, all over it. I can rub it off with a finger, but I assume it's also on the internal passages. This is a Nikki generator carburetor and I'm using Meguiars Super Degreaser for the fluid.
For reference, a carb I did a few years ago came out great.
What's going on and how to prevent it?