We see here coil size obviously matters... I'm interested in the difference between actual coil diameter and effective coil diameter, since the effective coil diameter is what Jbot changed.
Also the core material matters. I'd assume the cheaper pickups use cheaper core material.
In tattoo machines if the core becomes a permanent magnet the electromagnet becomes much less efficient, requires more voltage to operate. I'd imagine in the pickup circuit the same would he true. Makes me wonder if the pickups that become erratic or fail are still electrically intact, but the core has magnetized causing bad behavior.
The time obviously matters too. Jbot's modification may simply be reducing the voltage being sent to the CDI enough that it keeps the CDI from overloading and becoming erratic.
Even if I never use the information I like to know the why of things. It may help me somewhere down the road.(edited)