Originally Posted by BillGrissom

If the prechambers don't protrude past the surface (hard to tell in photos), I wouldn't be afraid to sand it with 400 grit. Search for they youtube by OldSkoolFunk. You need a thick piece of glass to spray-adhesive the sandpaper to. I would jam paper in the oil passages first, to pull out debris with the paper. Chemicals which might help are "fireplace window soot remover" and phosphoric acid. The later removes oxides and slightly etches bare aluminum (good for painting). You can buy it for concrete etching at Home Depot, or is in some of the rust-remover sprays.