The worst one I've seen so far - Skyactiv D carbon build up : r/Justrolledintotheshop
Mazdaspeed 3 - Intake Valve Cleaning - YouTube
JTech Autoserv - Another one Mazda 3 Skyactiv visited for intake valve walnut blast to de-carbon. Rejuvenate the driveability like new car. Rm480, walnut blast, incorporating with LiquiMoly carbon cleaning liquid. Refine
How to Clean Your Intake Valves | CorkSport Mazda Performance Blog