Unequal temperature is not good. It's a bit difficult to diagnose from the other side of a screen. Basicly 3 causes, fuel, spark or friction. There is several things you can check and the most common is an air leak that makes the air/fuel mixture lean to one cylinder. Other cause is the spark occur at the wrong moment. The third is friction. In any case the fault needs to be pinned down as soon as possible or a expensive breakdown is likely to happen sooner than later.
The spark in it self has nothing to do with the combustion temperature, it only makes the air/fuel mixture ignite, basicly the higher voltage you can induce between the gap, the easier a lean or turbulent mixture can start to burn. But our flatheads has a very rich air mixture compared to modern engines. A magneto of the sort that we use on our Indians gives about 12-15'000volts. Racing magnetos can make way more. Modern car ignitions with modern solid state technique of today's standard makes 25- up to 40'000 volts or even more. But the spark also needs amperage, or volume of electricity, to make a fat juicy spark with a long duration and a magneto is very good at that where solid state ignitions is poorer in that department. Flatheads likes a fat juicy spark.
NGK is listed with a very wide temperature range, in lists A6 and AB6 is equalled with Champion D16-D21-D9. The difference between A6 and AB6 is only the size of the hexagon key grip. Recommended Champion heat range is D16 or the hotter D14. A hotter range for the NGK is numbered lower (AB5 in this case) but I haven't found any reason to use a hotter plug.
A magneto is depending on the speed that the magneto turns, the faster it turns, the stronger the spark. The advantage with distributor ignition is that makes full strength spark from the beginning and that can make the engine start easier.
