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Originally Posted by nUcLeArEnVoY
Had your same exact issue with my 1979 Trans Am when I first got it. Can you actually hear or feel a higher RPM when it hangs, does your oil pressure go up to coincide?
My idle would hang up around the same amount, especially after coming from a stop from a long cruise in 4th gear. I did everything - heavy weight advance springs, replaced all vacuum lines, even the intake gasket; popped my carb open more times than I can count, had it sent to QuadrajetPower for primary rebushiny and secondary linkage recalibrating, made sure throttle plates were returning to where they should and that the secondaries were shut at idle, made sure it wasn't my AC idle solenoid.... everything.
Ended up concluding it was just a whacky tach lol. When I took an RPM-reading timing light to it after a long drive, even after it "hanged" a bit on the tach, the RPM still read the same as what I had set it to. So now, I just look at my oil pressure gauge now to verify I'm at my proper idle. 750ish RPMs for me is exactly 40PSI right in the middle when I'm warmed up. Hasn't bothered me since.
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Definitely not just the tach. Confirmed it by physically moving the secondary shaft. Everything else coincides with the RPM increase.