Here are the spark plugs. I lied, they are not all the same. They are ordered 1-4 from left to right. 1 and 2 are the ones I said clearly affected the idle speed, 3 and 4 are the ones I said have only a subtle effect on idle speed.
I would swear I've seen the black deposit on plug 1 come and go.
What do these plugs mean?
I've changed all the electrical components at least once, although I never saw any visible problems. Does this photo and my observations of idle speed result in favoring an electronic distributor upgrade? Yes, the current distributor has points. EDIT: no it doesn't. The distributor is mechanical, the points are electrical.