This sounds silly, but might check to be sure the oil fill cap is securely tightened and not cracked, and that all the openings associated with crankcase ventilation are not leaking.
Under normal conditions, the crankcase is subject to negative pressure, i.e., a vacuum. You can check this by making sure the engine is warmed up and at operating temperature. Then just unscrew the oil cap. You should notice a rougher idle. When you place your palm over the opening where the cap was, you should be able to feel a vacuum. If you feel air blowing out the opening or you can see crankcase gasses venting , then you likely have some blow-by. Blow-by can foul plugs and dilute the incoming AF mixture ratio. Either situation can a cause of a rough idle. No amount of tinkering with the mixture screws will affect this problem. Additionally, the ECU is not designed to take into consideration the 'extra' air (blow-by gasses) coming into the cylinders.
If you don't notice a rougher idle than before removing the cap but also don't see any venting, you may have a vacuum leak at some other location.
Mike in AR
