... My money is on the cap and rotor, but new plugs and wires every couple of years are a good idea regardless.
... and yeah, 95% of carburation problems are ignition. 95% of the time, that advice works every single time. This advice and a dollar will buy a small coffee at any McDonald's in America-- you can take that to the bank!
Green corrosion in the cap? Cha-ching!
For today, scrape the corrosion off with a flat-bladed screwdriver (the 'ol hick-town tune-up), and I'll bet it runs better.
Get a new one (and rotor), and you can buy me that coffee in the event you ever meet me. As an aside: at some point, you're going to have to make some provision or other to be able to easily remove #1 and #3 plugs. Having an ignition problem, and not looking at the plugs is like trying to read a book written in Mandarin, with the lights off, and your glasses on the night-stand.