Which magneto fires which spark plugs on my Lycoming engine?
Depends on the part number of the harness being used. The most reliable approach is to simply trace a couple of the harness leads. In most cases, the left mag will fire the bottom left plugs (2 and 4 on the Lycoming) and the top right plugs (1 and 3 on the Lycoming). The right mag fires the top left and bottom right plugs. The reason they are staggered (half top and half bottom) is to provide a more equal mag drop during a mag test. If they were all top or all bottom, there would be a larger RPM spread on the mag test.
As an interesting aside, homebuilt planes often use one mag and one electronic ignition. They are usually configured so that the mag fires all the top plugs, and the EI fires all the bottom plugs. There is always a large difference in mag drop anyway, since the EI usually provides automatic spark advance. Using the EI on the lower plugs, along with large-gap automotive spark plugs and large-diameter plug wires, provides a very large, long, and hot spark. Starting is usually immediate and fouling almost non-existent, despite the bottom-mounted spark plugs.