Not a theory! This method was originally taught to me by a GM engineer (now a rocket scientist for Martin Marietta).

There may be no vac advance on the '73, but there should be mechanical advance. I seriously doubt the D-Jet computer was sophisticated enough to handle engine timing. Try the above suggested method first. You should see an advance in timing as rpms increase.

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'01 SLK320, '79 450SL & '01 C320 -- What? 3 Mercedes? I am DEFINITELY crazy!!!