The IABs have a greater effect with higher air flow. However, the bigger IAB may also delay transition circuit flow. I don't consider myself an expert by any measure, but my thought would be to decrease the IAB size some since it is an easy change, and see how it affects the off-idle transition and low RPM AFR. If that doesn't adequately improve it, then come up a bit on the IFR.
I've been tinkering with my QF Q850PV; it has been rich all over the range (admittedly it is sold as a race carb), but I'm slowly getting closer to improving its street manners.
