If you do the full de-smog ... then you block off the connector to the the intake manifold (see following pics).
By doing that, you are changing how much air is getting into the intake manifold. That alone would be cause enough for needing to adjust the idle.
First Mr. E-Ticket, I must thank you for your generosity in responding not only to my post, but your outstanding knowledge sharing on your EXC 350 post. Tons of useful information! I have read it in the past, end to end!
This is very interesting...
On page 45, our friend Zuber says:
"The charcoal cannister/tank vent (closed tank system) has only one electical connection to the ECU, that's the air valve under the throttle body. The ECU does not detect if this is connected, like on the v-twins. It only sends power to this valve at certain rpm/throttle settings to vent out the charcoal cannister."
Maybe the valve is open at idle which is a leaning condition which tends to raise the idle speed and plugging the hole enriches the mixture by allowing less air, therefore tending to lower the idle, requiring turning out the idle screw? People talk about flooding with poor starting/idling after taking a left side fall. This would support that the valve is open at idle. If my premise is wrong that the valve is open at idle, then the logic falls apart and I can't make sense of why the idle would go up with disconnecting and plugging.
Also, if the valve is open at any condition off idle and we've disconnected the solenoid and plugged the hole, then the engine is getting less air than the ECU thinks and it's management might be off a bit? With less air than it thinks, it might tend to be rich? Any concerns or does anyone know that the valve always closed off idle?