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Need Help, Stuck SU piston


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Hey everyone,


I'm having trouble with my 2nd hand 1971 4 screw SU carbs that I am in the process of rebuilding. They are not yet installed and are currently on my bench.


The Problem


The piston ends up freezing in position when I install it into the carb body and screw on the Bell chamber. It stays closed the worst in the closed position

but also stay in the open position when it does get moved.




When i place the piston into either the chamber OR the body, it moves freely.

Being the guy that I am, I decided to eliminate as many problems as possible so I swapped components (piston, chamber/dome, nozzle, needle) from the front and rear carbs and

the problem persisted on only the front carb.


With as many SU carb owners that there are, someone has to have encountered this problem. If not, I'd love a helping hand on troubleshooting.


- So gentlemen, What problems can there be and how can i fix them?

- I was thinking it could be the nozzle (but i had that replaced)

- Maybe my jet retaining nut is bad?

- Warped carb body?

- Possibly oversized needle?

- Needle offset. (but works with other carb body?)


Thanks for your time,



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