I'll just throw a couple of things out there, that I'm sure you have already checked. These are for future readers using the search button.
1) the T-port sync: If the butterflies end up too far open, the Vacuum Advance port may be activated pulling in some timing, which will always increase the idle speed. The toe-tap may be just enough to drop the signal out.
Nah, don't think this is it because it's like 50-75 rpm and if it did pull some in this engine with it's timing settings would idle slower I think from too much.
2)the t-port sync: if the timing is too far advanced, then the throttle plates may park very closed to fully closed. In this position, the idle will be very sensitive to very small changes in throttle parking. It will usually be accompanied by a hesitation at throttle tip-in.
No throttle tip in or hesitation at all, and zero signs or sounds of too advanced. (I checked AFR and plugs both before satisfied)
3) the T-port sync; If the butterflies are open a lot too far , the transfers may not want to quit. This usually manifests as the rpm taking an abnormally long time to return to the preset idle. But it usually does return to idle after some 2 or 3 seconds. Occasionally tho, it just hangs. The proof of this situation is to kill the ignition for a second and reconnect at just below the preset idle speed. If it then remains at the preset, that is the proof.
The butterflies come against their bores pretty much at the same exact time the Idle speed screw contacts the stop, and the engines returns to idle immediately upon letting off.
Also when it hangs up it's got about 1/1000000 of and air gap betwwen the screw tip and the stop. (I checked) :D
4) the weights in the dizzy can hang up when very light springs are used.Extra advance means extra idlerpm.
Checked that also and not the case (Idle base timing is solid as a rock up to about 1500 before it starts to move.)
5) PCV. if the PCV does not park accurately, it can show up at idle.The PCV at this point is a controlled vacuum leak. If it doesn't park the same everytime, the idle speed will vary.This, however does not usually respond to a toe-tap. Exactly
6) linkages. Sometimes the links on the passenger side, make trouble. Especially the one to the secondary butterflies.There is usually a clearance spec for this link that is supposed to be reset every time the primary cracking screw is adjusted.