[Bob Ashlock]

Answering your earlier question about the 'runs fine for 30 min' ... I was trying to start off with the basic stuff first, checking power integrity and points as you initially commented that the tach was jumping around. Does the tach actually behave 'fine' for the first 30 min and THEN when the problem begins does the tach jump come into play at the same time? Is that a kind of 'violent' jumping or just a 'dipping' as the engine starts missing? One of the most common CDI failures is the discharge capacitor shorting or opening after the unit heats up. Of course this kills the spark, and the tach is going to start changing as the rpms come down. I think a checkout of the CDI is in order and there are others on this forum that have bench test setups to do this, as I also do. Running hot on a bench test usually exposes this capacitor problem right away. And then of course you still have what appears to be an alternator issue to contend with. The suggestion by others to monitor the 12V is important because if your alternator is going berserk and putting out higher voltage or the dc voltage is carrying a large ac component (bad diode?) then the CDI unit will be dissipating more power, getting hotter than usual, etc., all aggravations of the problem.