I hate to flog a dying point, but the Scr of that engine with open-chamber heads, is likely not gonna exceed 8.5/1
With a lopey cam, having an Ica around say 66* the cylinder pressure might already be in the basement... like 125psi; which means the vacuum signal to the venturis is gonna be ........... um, sluggish.
If it was mine, I'd be doing a compression test ASAP.
If it really is down around 125psi, I'd be rethinking a lotta things.