Based on a simple schematic for a single coil, no trigger CDI system I found on the net:
Here, the triggering is done at the crossing from positive to negative, discharging the cap. Simple enough.
To advance/retard the triggering timing, i was thinking of detecting the point at which the positive cycle
starts to fall and feed this into a uC so that it could control whether to fire the SCR right at that moment or
delays it:
Would this work? I should use an opto-coupler right after the PNP.
Any problems i'm not seeing?

 

To get the maximum possible spark voltage, the SCR has to be triggered at the peak of the negative cycle. This means that I would have to adjust the position of the stator. Since the negative cycle isnt being used but just to trigger the SCR, I figure, why not harvest and regulate it to charge a battery.