Can’t set ignition timing correctly

I’m having trouble setting the timing correctly. I’ll set the left cylinder perfectly, lock it down, then check it again. Still dead on. I set the right with the same process. Get it perfect, lock it, and check it. When I get the right cylinder timed perfectly, the left cylinder timing changes.

I have a CMC electronic ignition.

I’ve adjusted the cam tensioner. I’ve tried cleaning, lubricating, and tightening the springs on the mechanical advance.

What else could cause my issue?