Is this carbon buildup? And how can I fix this?

Mattax said
Yup.
Only way for burning to occur in the intake passages is for something to set off the fuel before the intake valve is fully closed.
So either valves not closing in time or ignition is too early for that rpm and load.

Or an exhaust lobe is flat and once the fuel mixture is set off the only place it has to go is back out the intake valve during overlap.