My way of doing things is to alter one thing at a time even takes notes and photos along the way and taking time out before it starts to get to you I was on the edge of scraping a 1951 black shadow some time ago even a to the stage of putting a hammer though the case and getting the grinder out and cutting into little bits but found having breaks even taking a day off the problem helps an awful you will get there in the end but do it in little baby steps.Not likely anything to do with the carb.
This is classic capacitor failure of an old magneto. Two solutions.
1) take the mag off and have it rebuilt
2) replace the mag with something modern
If you are not running a magneto what ignition do you have
Robert
PM me if you want and we can make contact. I am in Vancouver. I won't put contact details on the public forum.
before rushing off to throw money away take a long look at the problem and write a check list then do one thing a day at the time you have a lot of support on this web site