I agree with Vitesse.
Sounds like a carb issue - I suspect it could be a clogged pilot jet on the side that is having the issue. You may have to pull the carbs off or try and drop the bowl of the one side and get to the pilot jet. As prescribed by Vitesse - some Sea Foam cleaner it the tank [ might] clean out the pilot circuit. Works for some people on occasion.
Good luck & report back your findings.
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