If its a copy of a Honda then under the carb is a round cover. Undo the cover and remove and inside you will see the jet (you have to look upwards from underneath). Check it's not blocked by undoing and removing. Clean it all out and reassemble.
The screw on the carb doesn't really adjust the mixture to much its an idle air screw which doesn't do anything above 1200 rpm or so
Also try putting in a plug from Hondas recommended range (a colder one if you can get it)
Out if interest are you running the full foam/paper air filter?
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