Milt,
So, your question got me wondering.
What I found is some mixed points of view. What stands out is that an analog tach , like what we have as an original Buick part, needs a constant signal. HEI does not put out a constant wave signal. That in turn allows the analog tach to read fine at idle, but when RPMs spike the tach will go up then drop to nothing and then recover slowly.
There is a solution for this. It is a tach filter that is wired in line between the distributor and the tach wire.
From what I read, Accell and Mallory have a filter.
Loren