IFR & IAB Sizing Question

[CDN]

Question : when tuning the idle circuit does it mater how you arrive at the IAB / IAB combination that works best
At what pount do you increase IAB size to richen idle rather than just going down in size on IAB to get to where you want ??
Hope that makes sense

[CDN]

Damm Typo in body

Should say IFR not IAB / IAB in questions and should I change IFR

[GTO Geoff]

The IFR is not sized for idle fuel. Even with a very small IFR, there is always enough fuel for idle.
The IFR is sized [ increased usually ] for the T slot [ T= Transition, transition to main system ]. So that there is enough fuel for transition.
The IAB is sized to control the tapering off of the idle cct as the main system starts. A large IAB would end the idle cct at a lower rpm & could result in a flat spot, a 'hole' in the fuel curve.
Changing the IAB size has no significant contribution to the amount of air for the engine to idle. Idle air comes from air past the t/blades, from the exposed T slot, PCV, leakage past the t/shafts & any bypass air.

[cat herder]

Majority of the curb idle bleed air comes from the upper part of the t-slot, that's where the pressure differential (& air speed) is greatest. IAB contribution increases as more of the t-slot is uncovered and exposed to manifold pressure.

[CDN]

Thanks guys exactly the info I was looking for