What's for the "smoke fittings" in this muffler?
It is indeed so that you can generate smoke from the exhaust, typically for airshows. The general idea is that you introduce oil or specific liquids that have been created for specific colours to the hot muffler, which will create smoke. This is how skywriting works, the pilot can turn the system on and off, like lifting their pen from the paper.
If you're not using it, it does need to be plugged as otherwise exhaust (and sound!) will exit the hole rather than the correct exit at the end of the muffler. The &quot;fuel tubing&quot; mentioned in the manual is simply the same tubing used for fuel supply, as the diameter is suitable for it to slot over the nodule, and for the 8-32 screw to fill the inside diameter of the tubing completely, as the diagram indicates.