Definatly sounds like it's over fuling.
Perhaps somone here who has gone to bike carbs has a spare peirburg kicking arround. It's 10 years since I played with a 2e3 on a sierra, they have various things like accelerator pumps that may be stuck.
A full careful strip and clean may be all it needs.
One other thing... HAve you got a fuel return from the carb?
TT
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