Airbleed carbs have a hole that lets air in on lower throttle settings to affect the mixture. Turning the screw that partially blocks the air hole can richen or lean the mixture depending on how far the screw is into the hole.

We had a guy spend almost an entire day driving himself nuts with an airbleed carb. He'd never heard of one. And almost refused to believe they existed in spite of the fact he owned one and was shown what and why. And they're actually easier to tell what their initial setting is 'cause the hole is right there in front of you and the screw position is easy to understand.