Well, if its stock, you probably have a bing carb, the manual says that the carb jets are set stock at the factory for the engine size. Most people take that to mean that puch didnt want people to upgrade it trying to get speed, which is partly true, but it also means that the carb is a little to big for a smaller jet,
Meaning, that when you give it full throttle from nothing, the gas probably fills the carb faster than the jet can pour it into the engine, and it floods the carb and bogs out.
That is just my theory, I am no expert, but from reading the manual and having rebuilt a bing carb, that is what I would say the issue is.
I do not know why you down-jetted the carb, so I cannot really recommend a fix. From reading on here, I see that downjetting is usually a way to fix the symptoms of another issue (commonly airflow), and If I were you I would try to figure out how to get it to run with the stock jet.
Hope that helps somehow,