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.

Sticky pistons on 4-screw round tops. Not ideal. About the only things that would account for that would be a) severely misaligned nozzles underneath or b) mismatched domes/pistons.

You can check for the mismatching between the domes and pistons by taking the domes off, pulling the pistons out of the carbs and then sliding the pistons into the domes and see if they move smooth and free (while the dome and piston are in your hand, not mounted to the carb body).

And you can check for misaligned nozzles/needles by either removing the needles out of the pistons, or by loosening up the nozzle alignment nuts on the under sides the carbs and seeing if the pistons move free while the nozzles are way loose.