Mike Cross
Not getting a mag drop when switching one of the mags off could also mean that the magneto was not working in the first place. Either way, your course of action in shutting down is correct. However, (and I do not necessarily advocate this), to see what is actually wrong try switching the other mag off. If the engine keeps running but shows a drop in RPM then the first mag was live. If the engine stops then the first mag was not working. I am not a fan of this suggestion as it leaves fuel in the cylinders. However it does aid in the diagnosis.
M1.1