This is just an idea and I hate commenting on this as I know little about your carbs, but running with choke on, or hand choking it (blocking the intake) to get it to run right, to me indicates the same kind of vacuum/compression loss. In chainsaw stuff I'm more familiar with, it'd be an indicator that there's a compression issue, like rings are bad leading to no vacuum through the carb and you're opening that butterfly up and there's now less vacuum being created through the carb and it dies. Your bikes behavior is identical to my last weedeater that I had to rebuild with a new piston and ring set. Compression or leakdown test maybe if it's not terribly inconvenient?
A sudden change means something suddenly changed; I don't imagine this was a simple jet size problem. Although I could see trash in the carb doing it too.
