1981 QT50 Bogging under load

Hi i purchased a yamahopper qt50 today test riding it it rode ok and after about an hour of riding i turned it off for a second started it again and it was bogging and sputtering under load. I could rev the balls out of it with wheel off ground but if i sat on it it would bog sputter and die. Not sure what to do cheers

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1981 QT50 Bogging under load