I wonder if you may have a sticky slide or damaged diaphram. The problem sounds more like fuel to me since it is only the one cyl. and it happens at about 3500 rpm and then clears out after that. Did you check to see if that is more or less what it does all the time or only check once? I would think a bad coil would give you a problem at other rpms too. I would pull the top off the right carb and check that the needle is seated right and the diaphram is in good shape and that the slide moves freely.

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