I know I had some issues running a 4.8 amp Mallory Super Mag III and my boss had the same issues running an MSD 44 amp Mag.
You’d get a strange miss. Never the same RPM. I forget who I called but whoever it was said we needed to run a ground off both heads to the chassis because one ground wasn’t doing it.
I thought he was full of crap, but the extra ground cleaned it all up.
I’d like to know why the magneto needed that extra ground. I still ground all my big battery fired ignitions to both heads. I’m a chicken like that.
