Here's a link to a California shop that does it. They don't really discuss the process, but I found the article explaining why it is needed to be informative.
https://sprint-motorsports.com/maintenance/valve-decarbonizing/
I also spoke with Chris Rose from Continental Carbonic Products, Inc. in Decatur Illinois who indicated, as does this article, that the machine costs are the problem. Chris indicated that his organization provides the machines but is getting out of the rental business side and will will only be selling them going forward. He indicated that a rental could easily cost $40,000 for a week. Locally, Imola Motorsports responded to my inquiry to say they clean intake valves, but not with dry ice. There are apparently two organizations in the Minneapolis metro area indicating that they provide dry ice blasting, but neither has responded to me and seem to be designed to do big cleaning jobs. I haven't cleaned intake valves before, so I'm only guessing, but it sure seems like using a blasting medium that doesn't have any residue would make the process a lot easier. Possibly allowing for the valves to be in any position.
Have you done the walnut shell method?
