Hmm, not quite sure just where that passage leads to, or if it is directed to inside the upper end of the exhaust housing.  The 10/18hp had a little bleed hole right in the cylinder cooling jacket, in the area of your photo that is painted with that dark colored sealer/lyfanite.  I don’t see anything there, but it would be worth a few minutes to probe around in an effort to find a possible bleed hole.  Please don’t consider just adding a bleed hole in that area on your powerhead, it is very possible that an errant spray in that aread could be sucked back inside the exhaust ports.  Scrape that loose carbon away, perhaps you will find a bleed hole.