I haven't been following this thread, but I just went back and read some posts. This engine is problematic with two different carbs. That would make me change out all the ignition components as the next test.....

I had one engine that just was a bear to start. Took forever and a lot of finicking to get it to start. Bill from CH told me to try 6 volts on the ignition instead of 4.8, and then it ran perfectly. It was a weak ignition. An edgy hall sensor will drive you nuts as well...

AV8TOR