if the idle mix screws are not making a difference, are you running on the main system at idle?
could it be idling away merrily on a feed from the main circuit hence the idle circuit adjustment has no imapct.
the driver is often not enough initial timing for todays ethanol petrol so the throttle needs to be open to achieve a decent idle speed and enough idle torque to turn over the converter/trans in neutral and not stall as you move into drive
not so sure that creates a stumble but it certianly is one of many ways that the idle mix screws become ineffective.
i have no PCV on mine due to weber carbs and i must admit my rear main seal doesn't do its job as well as it used to, conincidence or cause?