The lower IAB's can have a major impact on idle tuning with some engine combinations.
For "mild" engines making good vacuum you may be fine leaving them at .075" as Kenth mentions.
For engines that use cams with a lot of overlap, tight LSA, or low vacuum at idle you may not be able to effectively tune them without making the lower IAB's smaller. Keep in mind that the lower IAB's are the first place to "vent" the fuel supply passages to the mixture screws.
Creating a large leak there can reduce the signal so much up to the DCR's that you can't make the idle tubes and DCR's big enough to get good sensitivity from the mixture screws, especially if there is a pretty decent size Upper IAB venting the passages again.
I've tried tuning some pretty heavily cammed engines without making the Lower IAB's smaller and was partially successful, but it required huge idle tubes and DCR's, and even then the idle quality wasn't all that great.
I've had much better success with some set-ups that have low vacuum at idle by making both the Lower and Upper IAB's smaller vs huge idle tubes/DCR's.
Keep in mind here that some of these engines required this simply because the engine builder and/or owner made poor parts selections in terms of compression and camshaft. Any time you start pulling the LSA down, increasing overlap, and longer seat timing the static compression ratio becomes a pretty big player in how well the engine will idle. IF you find your engine needing a lot of timing and fuel at idle speed the cam simply isn't all that well chosen for what you are doing more times than not. I'm talking here about street engines.
For race engines designed to make big power in a narrower RPM range and where idle quality isn't a big deal narrow LSA and long seat timing cams are pretty much the norm and the end user expects low vacuum at idle, rough idle, stinky exhaust, poor street manners, etc........Cliff
