went up 2 size on main jets (56) and it runs great / how I need to do the dist work this weekend

Good to hear.Jet size affects all aspects of carb performance except maybe idle speed and mix.

  

I was running 54 primaries on my old Holley 390 cfm. I went to the Demon carb, and eventually bumped those up to 58s. There was a noticeable improvement.

I tried 58s on mine.Too big,ran rich.Every motor/carb combo works a little differently.

  

Yes. My 312 has a cam that's a bit livelier than stock, and ported ECZC heads. In additon to the primary jetting I also had to downsize a bit on the idle feed restrictors since the Demon came with .031s. Anyhow, finding that correct balance between lean/rich has required some tweaking. Better to err on the side of rich.