if truly at 1/4 throttle pos, your slide needles are barely supplying fuel to make a difference.
you're right in the mixture screw/pilot jet wheelhouse, so, go 1/2 turn RICHER on all mixture screws, ride test at 1/4 throttle pos, and if that doesnt get you the result you want, go to 40pilots and back to 3/4 turn on the mixture screws, and retest.
be sure to use OEM Keihin jets