Seafoam did a nice job on my Merc 7.5.
Ran nice to begin with, and idled a bit better after Seafoaming.
Followed directions from Seafoam.
Noticed a lot of the carbon on the tops of the pistons was still there.
Poured Seafoam into the cylinders and let sit over night (laying down, pistons up, and mid-stroke so that Seafoam did not leak down).
After approx. 12 hours, the Seafoam was dark colored from dissolving carbon.
Lightly Seafoamed as usual again, and this time approximately 60% of the carbon was removed from the tops of the pistons.