Ok, if 1/4 are rich, a possible culprit is the coil on that side. The spark plug caps should have a resistance reading of around 5k ohms. You can unscrew them from the plug wires and use a meter and measure them. If they are good, you can trim a very short it of the plug wire back and reinstall the stock caps to help produce a better spark.
I am believing the issue to be spark, not fuel because of the cylinder pairing. Which electronic ignition is it? And can you provide any history or detail on the bike? How long you have owned it, have you done any other service beyond the steps listed in your initial post?