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It does sound as it its getting too much fuel when you open the throttle. The engine is fitted with an overload stop which need to be set up. I've enclosed a copy of the parts book page and it's item 22.
Rings would be the first thing I'd look at.
Most likely is over fueling. If no luck with that try loading the engine up by putting it under load and run up hill to make the engine work hard and run it like you hate it while loaded. Possible glazed bore could also be considered.