I know you said it's clean, but many have said that before and found out they didn't do a good job the first time.
Since the carb would be apart, I'd look up the correct float level and measure it.
Then inspect the rubber diaphragm/piston/spring assembly. It must be supple. No holes or cracks, etc. I'd get an exploded view of the carb parts and compare them to what you have. Pay attention to the correct assembly order of all parts, especially the diaphragm asembly. Be sure the diaphragm is seated correctly too.
Inspect the throttle plate for smooth action.
Since you inherited the problem, the PO could have done anything to mess things up, or the bike may have sat awhile and the carbs needs a better cleaning or could have rubber parts in poor condition as I've said. It could have incorrect jets, etc.
Some of this stuff I mention wouldn't cause a "shut down" as you describe, but if you have the carb apart, do a 100% job. Sky is the limit here when picking up a bike with a problem.

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