intake manifold question

It is carbon, from the exhaust being forced back up through the center of the intake for warmup. This is the job of the heat exchanger on the passenger side exhaust manifold, to remain mostly closed until the thermocouple spring on the outside of it warms up, and holds the flapper open.

Of course on a true performance set up, you don't want heat under the carb, so you block those passages off. But you sacrifice cold engine driveability and and warm up when you do that.