DV seal issues (the copper seal) can cause idle roughness and perhaps other issues. DV o-rings can fail and allow leaking from the base of the DV holder. If you have ruled out the injectors and air-into-fuel-system leaks and fuel filters as possible sources of your issue, then re-building the DVs would be a good next step. The spring can also be replaced, they do weaken and get shorter.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC