I have a 1967 4 Cyl Volvo engine that needs an exhaust Manifold. Where might I find one, and are they hard to install? I am trying to save money, but want to do a good job. Also, would having a leak in the manifold cause a new plug to foul? I put new plugs in and took the boat out once, and it backfired twice. When I removed the plugs, one was burnt and had oil on it. then I realized I had a hole in the exhaust Manifold.