Ron, your reference to an MGB is a major insult to your Mercedes and to my ''B''.
As a present owner of an 1974 ''B'' and as a previous owner of 3 other''B's'' and ''A'', I know the the ''prince of darkness'' can be a real pain.
Sorry I can't help with your MB problem.
MGB were never designed to idle properly. It would seen a smooth idle was outside the capabilities of MGB engineers.
But on a nice summer sunday afternoon, I still prefer my old ''B'' for a nice country road drive.