Try to completely cover the carb with your hand and see if it dies.
If it don't die then you know you have a leak for sure...but it still could have a small one even if it does die. Maybe spray carb cleaner along the intake /head surface and see if the thing speeds up. I've also heard of people using propane to find a leak but never tried it. Could be under the carb too.
P.S. The idle screws on a Holley metering block control fuel not air unless I am misunderstanding your post.