You mean inside the intake ports? The runners?

 

The inside of the intake ports

 

that's carbon buildup.. likely due to poor quality petrol

take some brake clean and spray the inside of the intake ports then brush with a toothbrush or metal wire toothbrush (not actually meant for your teeth..I hope, do not do the block (or if you do, don't let any bits of carbon in)

 

How do you recommend cleaning the block ports?

 

Brake cleaner and toothbrush.