[Schulisco]

You can try to clean them by using an injection cleaner additive in the fuel at first try. The problem on these for CIS - they're mechanical valves opening with a pretty wide pressure range from 2,5-3,6bar / 35-52psi when new as Bosch/Porsche specified in the tech spec book for the 911. Problem is: They worn out mechanically over time. A refurbishment, as we know it from the Carrera injectors, is afaik not possible.
If the cleanup with the additive won't work, I personnaly would buy new ones from Bosch as I did when I worked my CIS. IIRC the average mileage lifetime of them is about 100.000mls... Adrian Streather mentioned in his book "Porsche 911 SC essential companion" on how to backflush them. Several threads already discussed this here as well. Never did it personally, the old injectors laying in the garage waiting for a try... Might work, might not. Don't know.
But again - it's always worth a try before buying new ones, no question.

Further reading:
https://forums.pelicanparts.com/porsche-911-technical-forum/399562-backflush-cis-injectors.html
https://forums.pelicanparts.com/porsche-911-technical-forum/403449-how-backflush-cis-injector.html

Many more, even videos:
https://www.google.com/search?q=Bosch+cis+injector+cleaning+backflush

The YT Channel "KlassikATS" has some of the best Porsche and CIS videos in the net: https://www.youtube.com/@klassikats, especially the video series on the 2,7 CIS engine: https://www.youtube.com/playlist?list=PLkWFofpgz2OgF9FdliKnlzuY0pygqRkMj. The basics are pretty the same to the 2.7l and 3.0l SC engines.

Thomas