If I understand, your vac advance is only giving 2-3 degrees. I don't know proper spec but I would expect 10 degrees or more from the vacuum.
Ported or manifold vacuum source?
Me too, so that is my ongoing tuning project right now. I've noticed I'm only getting 18mpg too, which used to be over 20mpg.
I'm using the stock manifold vacuum setup with check valve on the water neck too. My can is an Accel adjustable unit which is supposed to provide 20* of advance. Will be playing with the spring setting on that and checking my vacuum too. I've been waiting to get my wife on the pedal to help me check the full curve to see what my springs are providing.
I don't have a catalytic converter or EGR. I've seen recommendations for HEI advance to have 12* of vacuum advance when you have 32* to 36* of mechanical/initial advance by 2800 to 3200rpm. So I have a Crane adjustment cam for the vacuum, but I'm not even getting close to that.
