Great thread. Registered here because I've been searching for answers and this was the most informative place I found out there.
I don't currently have a quad myself, but my buddy has a '96 300 Fourtrax 2x4 that he's been having problems with. He's had a few people look at it and nothing, so he asked me to have a go.
No spark at all. So far I've tested all the wiring for both power and resistance (where applicable), replaced the coil and plug wire (new) and CDI box (also new), neutral safety switch, and of course the plug. Nothing. Only things I haven't replaced are the stator and regulator, stators are expensive just for a test.
Power to (and past) the ignition switch, neutral light on, oil light pops on for a few seconds, starter works fine.
I even put a meter on the hot coil wire while I cranked it, and seem to only be getting a peak 0.03-0.04 volts on pulse. After reading this thread I also tried unplugging the regulator and spun it over - nothing.
No spark at all at any point, not even a weak one. It would seem after reading this thread that there isn't much left I can try except replacing the regulator and/or stator, but at this point replacing more parts as a test doesn't seem like the best idea, and I don't have another unit to borrow parts from. Just wondered if anyone might have any thoughts on any more testing I could do without more parts swapping.