Any engine will run cleaner and better if you take the time to properly seal it. Buy high-temperature sealant such as Permatex HIGH-TEMP Red RTV Silicone Gasket Maker and apply it very sparingly to any metalto- metal surfaces where hot gases and oil might leak out. Here, sealant is applied between the exhaust flange and the manifold. Apply just enough so that you can see a tiny amount extrude as the parts are bolted down. This will form a tiny gasket that will keep the goo coming out the muffler, where it is supposed to exit.
Brad Willis, Torrington, CT