CARB EXPERTS ? IFR/IAB

can anyone school me on what occurs when the IFR is made smaller or bigger in relationship to the IAB .

i seem to have a really lean idle in a carb i sold to my buddy .


thanks

Increasing the size of idle fuel restrictor will fatten up idle/transition circuits , increasing idle air bleeds will lean out idle/transition circuits , increasing/decreasing by .003" will have a noticeable effect.

I agree with the above.

It may help to think of the IFR as the idle main jet. It meters the fuel on the idle circuit. Larger orifice = more fuel, smaller = less.

And think of the IAB as idle jets for the air. This time a larger orifice opening means more air in relation to the fuel, aka leaner, while a smaller orifice results in less air and a richer idle mixture.

You may use a little caution when messing with the air bleeds because they also affect when the main fuel circuit activates.

Do you have a wide-band O2? They help a lot not just to let you know mixture, but when each circuit comes and goes and what effect your changes have on each.

If you have any off-idle issues, don't forget about the transition slots.

If you already know all this, my apologies. I just hope to make you aware that idle and off-idle tuning is the most complicated because there are several circuits overlapping.

thanks for the info,
i swapped some billet metering blocks into my friends carb and the car would no longer idle.i had to go from 71 IAB to 55 IAB to ge it to idle . i see the IFR is smaller in the billet blocks than the IFR in thr stock blocks.

You can try some very small diameter wire in the IAB to see how much that helps, the other thing is the idle mixture screws and throttle blades opening, front and rear Let us know what ends up working