750 Edelbrock issue

To add to what our friend down under Bewy said, not enough initial timing can cause all of the things he outlined. Too much transfer slot exposure, causing it to be on the main circuit at idle and pulling extra fuel in through the boosters. It's getting extra fuel from SOMEWHERE. It's VERY easy to pull some extra initial timing in and see. I'd pull in 18 and not mess around and see if it improves. Here's a clue. If you start pulling in more timing and the engine RPM increases, there you go. It's tellin you it wants more. It costs nothing and you can put it right back where it was if id doesn't work. But I bet it will.

I also agree on his point to the fuel pressure. Holley carburetors will stand close to 10 PSI before they push the needles off the seats. Those floats have a lot of leverage.