On my 454 mag I am getting a stumble on light acceleration. It will do it all day long if I leave the throttle just above idle. It goes away as soon as I give it more throttle, roughly 20% is what it needs for the stumble to go away. I am thinking it is in the carb setup. It has a Edlebrock Marine 750. Whats the correct way to set these up. It was on the boat when I got it and it has always had this problem.

You need to experiment with different weight springs on the metering rods. You can get a calibration kit with an entire selection of various springs (they are color coded), a selection of different rods and jets from Eldebrock for about $25. You most likely need a heavier spring on each rod. It's easy to do.