Just do as seamus said and clean up the surfaces then do as Kaos said and run up your engine to operating temp. and then tighten up the screws. The heat of the engine expands the aluminum hotter then the steel screws so they tend to come loose. It's normal for pretty much all engines to loosen up the screws when new. Don't bother with the gasket. I have only one engine that I use the gasket on, the gasket is different then others and really keeps the exhaust from seaping from the muffler. I still tighten the screws when hot.