Dr C what holley 950 do you have? The 830 cfm version? That's what I have and it acts the same way. I finally settled on 70 iab, 28 hsb .081 on the T slot. .031 ifr down low. Its better not perfect. What I need is more idle timing. Without locking out the dis. 34 total 24 initial.
Sorry, didn't see your question.
I don't know exactly, it's a "hybrid" (QFT/Proform 950 body with 1.42" venturis, QFT baseplate with 1-3/4" butterflies, billet metering blocks, Holley Ultra HP fuel bowls).
The key seems to be a small enough IFR and a very small MAB (hsb). I have 16 cranking 27 initial 36 total, plus a vacuum advance hooked up to manifold vacuum (these blocks and baseplate don't have vacuum nipples at all). But 8" Hg at idle is not enough to provide any advance there. It could use more at idle (1200 rpm).
