The symptoms point to an inadequate fuel/air ratio so what are the possibilities? Partially clogged main jet, intake vacuum leak, fuel contamination, fuel level low in the bowl. Do you have a PCV system? If so, check it for leaks and proper valve operation.
However, there was one other thing you said:
Quote:
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The engine this spring has been toyed with a little in terms of . . . . . power tuning at idle.
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If by
tuning you mean adjusting the timing, there is a directive from Universal about power timing at idle but the preferred method by far is to power time at greater RPM with the advance fully engaged. My earlier power timing efforts were at idle but checking it carefully a few years ago with a timing light showed it was grossly advanced. Getting it right made a huge difference in performance.