I had this problem once, and tried swapping out the joints but they did not work. I had an idea that involved removing the couplers all together. I made a throttle shaft with two "drive legs", one to control the front carb and one for the rear. Add two threaded rods and ball joints per carb and your done. If you want a drawing with dimensions, I'm sure I can draw one up from my notes, if not, it's simple to measure using your old throttle shaft as a sample.
-- Sergio 1967 Sunbeam Alpine Series V 'AJNT 86' still working on it!\\\|/// \\ ~ ~ // (/ (at) (at) /) ~~~~~~~oOOo~(_)~oOOo~~~~~~~
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