[huddy]

Aircraft magnetos are usually timed a couple degrees apart. Part of the pretakeoff checks is to disable each mag individually and watch the tach. With a small displacement, carbureted Continental 4 banger at 1800 RPM you will typically see 150 RPM drop when running only on one magneto, and usually 50 RPM from one to the other. This tells me that two plugs firing with timing a bit apart gives you a more efficient fuel burn. I don't have experience with a Model A engine in an airplane but I would think it would work the same way, except, of course, the runup RPM would be somewhat lower. (One purpose of the runup test is to ensure that both mags are firing, and also to ensure that the "kill" switch is actually killing the spark on each mag, both of which are important to know).