How would I check to see if there is a partially clogged injector??? Or is that an area that I should be considering?

My thoughts are to crack the line at the injector of each cylinder and if the idle does not worsen significantly, then there may be a problem with that injector?????

Help please...

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1982 300CD (Veggie)
1975 240D
1986 190D (lost a bearing)
1982 240D (dead eng.)