If you use a drip irrigation needle valve you can vary the vacuum orifice size with a twist of the valve handle.
Here's a link to a photo of my setup in my 83 W126 300SD

http://www.peachparts.com/shopforum/attachments/diesel-discussion/141631d1491110554-81-300sd-hard-shut-down-needle-valve-place-standard-orifice-reducer.jpg

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022