I had the exact same symptoms when I drilled the airmixture screw cover and opened it to ~2 turns. Close that down until it behaves. Mine likes 1 turn, or even a bit less, but I changed to a #42 pilot jet (up from #40) as well. If you haven't rejet, START there!
https://www.chinariders.net/showthread.php?t=32177
Look at post #4 for links to rejetting info.
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