Values vary considerably depending on the make
and model of magneto and who wound it. However, if the reading is more than
7000 ohms, there is probably either a break in the HT winding, or a bad
connection between the live end of the HT winding and the slip-ring. If the
reading is less than 3000 ohms, it would be unusual, but is indicative of a
problem. In either case, the magneto needs a more complete overhaul.