I haven't done a plug test before, but aren't you supposed to run it at normal speed to do the test.
I wish I knew more about the carb and it's settings.
i don't know, much but my problem area was in the bottom throttle area which is only controlled by the pilot jet and the mixture screw, to drive it at half throttle would show me what my main jet and needle settings were. i downloaded some writeups on carberation and it helped a little, just so many variables.
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