Regarding seafoam or any other cleaners used to clean carbon...it really depends if the carbon is more of a stain/sludge or if it actually hardened. Because if it hardened - and I find that it usually will with time and heat cycles - you aren’t getting that off with seafoam, no matter how much you dump down in there. Heck, I’ve used CRC, sprayed an entire can directly on to hardened carbon on a valve and it did NOTHING. I then let the valve soak in an entire can of seafoam for two hours, and it did NOTHING to hardened carbon. So, then I took that valve, heated it up to engine operating temperature, with the seafoam solution soaked into it, and then sprayed another entire can of CRC intake cleaner directly on to that valve...and it did next to NOTHING. Maybe a few tiny little minuscule specs came off, but nothing worth talking about. 2 entire cans sprayed directly onto the carbon, and one bottle of seafoam soaked for two hours, and then heated up the valve to operating temp (lucky I didn’t blow myself up, surprisingly it only smoked).
Unfortunately if it is hardened carbon you’ll need to replace it or scrape it.