I suggest pulling the carb completely apart, pulling all the jets and cleaning everything real well with carb cleaner, using compressed air to blow everything dry.
Make a note of the jetting that's in the carb now, including pilot, main, needle type and clip position on the needle, and the number on the slide. Check the float height and make sure it's within spec.
Clean and oil the air filter. Pull the stator cover off and make sure it's clean and dry, and verify the timing is within spec. Put everything back together and install a fresh plug, then see if you still have the problem. Let us know what you come up with.
A service manual will be helpful also.