Surging idle with Q'jet

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Freshly rebuilt Q'jet with manual choke and i took great care in the process - well, for as much as I know. SB 350 starts runs and revs just fine, and will idle at fairly low revs, but there is a slight "surging" action (not bumpy or stumbling) that I cannot eliminate with idle adjust screws or fuel pressure regulator. I have carefully checked all vacuum inlets and believe there is no air sucking in topside.

I have seen mention of too much opening at secondary (or primary?) throttle plates as a cause of idle issues. Not sure how this would have developed because I changed nothing there, but maybe this is an issue. Is there a quick and good test to see if this is a cause? Can anyone point to some info on how to check and correct such an issue with adjustment - preferably with carb as mounted?

Thanks for any help.