New information to ponder. Everyone running 48 IDA carbs has always heard that the idle mixture screws need to be 3/4 of a turn out or your idle jetting is wrong. When I run at 3/4 of a turn I have extreme backfires at decel and not very good accel. Recently I have discovered that my car runs very well with the screws turned out to 1 1/4 to 1 1/2 turns and the backfires have gone away.
Here is my current thinking - I am running the new Spanish manufactured carbs. I had access to a set of old Italian made carbs and I compared the idle mixture screws. Turns out that the new idle mixture screws are a larger diameter than the older ones. This would mean that there is less space in the idle mixture channels with the new screws, so it makes sense that you would need them turned out further so that the volume of mixture would be the same as that with the older screws. I'm not sure this is totally correct, but it is my story and I am sticking with it!
