I have the Promax billet block on my center carb, and was getting wide swings in idle A?F ratios. The IFRs were located up above the fuel level on that metering block.
I read that moving them to the bottom of the passage below the fuel level would stabilize that. A fact that was discovered in WW1 airplane carburetor research. It worked, and my idle fuel level stabilized. I also went with a .035 restriction in place of the .033.
