Exhausts manifolds are FAIRLY easy to test and should be done yearly(my opinion). Plug 1" heater hose on the manifold with a 1" wooden dowel inside hose. Position the manifold so that you can pour acetone into it the water or anti-freeze cavity and let sit for a few minutes. You can then shine light into the fire chamber to examine for wet spots. I am sure that gasoline will work just as well to find any leak/ crack, and it is much cheaper. Risers, for me, took a little fabrication of a piece of 1/4" aluminum plate with gasket material glued to it. Be sure to block off 1" nipple on riser. With a good jig-saw, you can slowly but surly cut aluminum plate to fit Mercruiser gasket and riser hole pattern. Once completed and positioned, pour acetone into it vertically. Wait a few minutes and check for wet spots. Notice how I have riser positioned. Don't forget to plug hose/nipple. I thought about fabricating a pressure testing device for manifolds, but due to their design, it would be much more difficult fabricating a pressure testing device for risers. I hope this helps. Iboats may sell the cap that I fabricated. Attached are photos.
