In my experience, bad gas will act lean rather than rich. It sounds like changing the ignition system is giving you a better spark to burn the A/F. Good. I am assuming you changed your plugs as well, right?

Also, with the light, what is your timing set at now that it seems to run better? How far did you have to advance it to get "much better success" by ear? If you're still having problems, that double pumper may need a tune-up, time to break out the vaccuum guage and tach after your timing is set (with the light).