IMO it would be difficult to damage a valve stem using any hand tool except a hammer. I'd use a scraper made from hardwood that you could shape as needed. Make sure both valves are completely closed, scrape away and blow it out with compressed air.

Look into adding a catch can to the breather system to reduce intake buildup.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer