The vacuum advance on my HEI doesn't seem to be giving me any more of an advance than about 6 degrees. So if I set initial at 10 I don't get any more than 15ish under full advance and my motor just goes whoooaaaa....I have a MSD cap and rotor and an Accel supercoil and springs and weights in it with the mid springs. Any ideas?

 

Well vacuum advance is only beneficial under a low load setting, because that's when manifold vacuum is it's highest, but when the throttle plate opens up more and more the pressure in the manifold becomes "less negative", if you will, and advance tops out sort of.

So that's where centrifugal advance with weights comes into play, and the speed of the distributor gear throws the weights out via centrifugal force thus advancing the timing, so if you're having a problem when you step on it, check the weights and their movement, make sure they're not restricted, make sure the spring tension is correct.

Hope this helps.