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Also a remote possibility that the intake rubbers are leaking and someone has responded incorrectly by putting in larger idle jets, giving you a mix of two problems. Can you assess for air leaks?
The idle settings are curious certainly, at three turns out it would ordinarily be going toward lean and possibly starting to miss a little. It suggests that the idle jets are larger than need be as you can't lean it off enough, but would explain why it's set on arrival at 2 turn out which would probably be better for it.Too rich idle would give you that "off choke early" necessity, which seems to tally. Looks like you'd need to pull a float bowl and use a mirror to see if you can identify the idle jet size.But it would usually "quench" the revs quickly, for which I'd suspect something else. You've checked the timing, but did you assess if the advance/ retard is also dropping back fully when you close the throttle? If it's holding onto advance that could give you the hanging revs temporarily until it closed the mech down.Also a remote possibility that the intake rubbers are leaking and someone has responded incorrectly by putting in larger idle jets, giving you a mix of two problems. Can you assess for air leaks?