Round top SU carb piston binding

I’m rebuilding my round top SU carbs on my 1971 240z and after everything was cleaned and put back together one of the pistons keeps binding in the dash pot when the top is screwed down. I’ve switched tops and the same thing happens. The jet and needle don’t bind at all and the piston slides freely with the top not screwed down. I’ve tried tightening the screws gradually and making sure the piston slides freely as I go but every time the screws are fully tightened the piston gets stuck and won’t fully drop. Any suggestions? Do the screws need to be fully tightened for the carb to function or can they be snug without causing issue? The carbs are 4 screw tops.