We had the same question a few weeks back. I believe the problem was a bad ground or bad starter solenoid.

That's what I've heard it was too.  A retard for the timing at startup.

If you change the flywheel to the old 4 speed one, it will work fine.

There is a timing difference.  I have no idea how much or which way, just that it's different.

You probably won't find a diagram anywhere.  The manufacturers like to keep that stuff as secret as possible.  Otherwise, they couldn't charge hundreds of dollars for each different one. The chinese would copy them all and sell them for next to nothing or just make up an adapter cable to run a standard chinese cdi.