Question about weak spark

[acca]

Hello
I'm suspecting my ignition coil to be faulty
The spark is purple, is that normal ?
My timing light doesn't flash at every RPM, is that normal ?

I have been "tuning" my engine without lambda sensor for years and i had no ignition problems.

After adding a lambda sensor, i realized that previous mixtures were very lean, so i corrected that.

Now, my engine works better but i have misfires sometimes (sometimes it stops the engine instantaneously). It happens very randomly

I'd like to know if a weak spark can be sufficient with a lean mixture but not with a rich one ?

Thanks