A smaller IAB will richen the upper rpm portion of the idle circuit ie 2000rpm. Your car cleans up at 2500 due to the primary being all the way in. Put .067 IABs in at least the front and readjust idle mixture for best idle. On a race car I set idle mixture to the fat side since you really don't idle much and the motor is run much colder than a street car.
The IAB controls the air allowed in the idle well, the IFR the amount of fuel, the idle mixture screw controls how much of the idle air/fuel mixture goes to the idle and transition slot.
