Originally Posted by supershaft
Isn't it funny what some people hear and read? I have heard and read what I posted up above all over the place. I have experienced it all over the place too. It should be a common fact: The more pressure in a cylinder, the more energy it takes to get a good spark across the spark plug gap. It makes perfect sense in my mind. Especially since I run into this fact all the time working on bikes.
The primary ignition circuit can be very much effected by heat and vibration. That is in part controlled by engine temp and load (what gear it's in). The secondary circuit is very much effected by BMEP as well which is very much effected by load/temp/pressure.
The most common failure in the old Lucas K2F Magnetos (used on British built twins like Norton Atlas, Royal Enfield Interceptor, Matchless...) is for a short to develop within the armature coil.
The bike will idle, but dies when you try to apply any power at all.
A static test with a spark plug shorted out against the cylinder head will always show a nice blue spark, but when the plugs are screwed back into the engine and you put the bike in gear and let out the clutch, it dies.
I've had this happen to two different magnetos in my Enfields over the years.