[geezer#99]

That means you’re hooked to ported vacuum.
Getting enough vacuum to add 4 degrees timing tells you that the idle screw is adjusted up too much. Carb is idling on the power circuit. Idles rich.
Hook the hose to the right port. Idle will increase. Then adjust the screw back out.
That could be the cause of the stumble.
That should help ir run better.
Then try more initial.
Bump it up to 12.
See what that does.

Are you still using the original valve springs?
Low power is a symptom of weak springs.