They don't have carbs, But most have a one way ball valve around the base, above the water level to allow you to push stale smoke out of it if necessary with out shooting it up the pipe stem......
ie...when you suck the ball gets sucked down blocking A hole allowing a vacuum.... when you blow into it the ball lifts a bit to let the stale smoke out
My Avatar is custom By Me.... I had the little Display Poster That sits atop the rack of shirts At shirt Hell lol (not the shirt itself) ...... I took a picture of it