One important question, did this just start happening, or was something, even seemingly insignificant, changed? Why was the carb off, to fix this problem, or another reason?
Agreed with what the others have said - but I do think it's important that it runs smoother under load.
That generally means the governor is opening your carb further to keep the same RPM level.
So, if opening the carb smooths the engnine out -at that same rpm level, it makes me think the problem is carb related.
Cleaning a carb thoroughly doesn't necessarily mean it's going to work perfectly.
I'd go through what others have said, and check/fix the easy stuff, especially the points, and checking the advance.
But I wouldn't cross the carb off the list as a known good.
I'm not familiar with the carbs on the 400, but I'm assuming there are some tolerances here or there worth checking on it.
If you come back to the carb, I'm sure others here could give you some pointers one what too look at.
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