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.

You say: switched tops but maybe you must switch top and piston on the other bottom part..

btw always keep the parts of 2 carbs ALWAYS apart.. they look the same but are very fine tuned parts..

3 hours ago, dutchzcarguy said:

You say: switched tops but maybe you must switch top and piston on the other bottom part..

btw always keep the parts of 2 carbs ALWAYS apart.. they look the same but are very fine tuned parts..

Correct……Always do one carb at a time….piston and dome are precision matched. One more thing, make sure the needles haven’t been bent.

If you do one at the time you have a (mirrored) example there to see how it sits/comes together again.. and... ALWAYS make pictures BEFORE taking apart.. it's so easy to miss that sequence of the little rings/parts on there!

RULE NUMBER ONE: MAKE PICTURES OF EVERYTHING YOUR GONNA RESTORE!

9 times out of 10 the problem on 4-screws is a misaligned nozzle, particularly if the large retaining nut underneath was loosened for some reason. There is a tool to help center the nozzle. If you take the needle out of the piston you insert the tool instead, which centers the nozzle in the casting. While the tool is in the piston and the piston pushed down in the chamber, tighten the large nut in place. Once the nut is tight, remove the piston and remove the tool. Before you re-insert the needle, chuck it in a drill motor and spin it to make sure it is perfectly straight. If it is, reinsert the needle in the piston and screw the dome back on. I'll take a photo of my tool and post it here shortly.

Here is the SU tuning kit mentioned in post above. I just checked on eBay and they are available for $40. The small aluminum fitting laying on top of the gray package is the one I was referring to.