So seems like consensus is leaning toward breather valve issue:

Does anyone have diagram of things that may need to be replaced (specific tubes/lines).

Lastly, I will note co-incidentally: when I bought car, PO had a fuel cap on that did not fit (figured PO drove off one day and lost original). I did buy a OEM MB cap later on, and didn't think anything of it....humm... could misfitting cap allow effectively funtion as a breather?

And I will order fuel strainer (wonder what tools I will need, hopefully nothing too specific..)

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Current:
1971 Mercedes Benz 250 (Euro Spec)
1972 Mercedes Benz 250 (US Spec)
Past:
1972 Mercedes Benz 280