I put a 1/2" carb spacer on my car and know I have a little stumble when I floor it. I assume it is what you call off idle stumble. If I am cruising at a steady speed and floor it, it stumbles and the picks up real quick. It ran great before the carb spacer. I am not sure if the spacer would make it lean due to more atomization of the fuel or what. Hopefully somebody here has the answer. I think I have 35 squirters all around I will have to check for sure. Maybe I need to jet up. i did notice before I put the carb spacer on that the secondary squirter really doesn't squirt the fuel but kind of pours it in at a slow steady pace. Maybe this squirter is to big.

First check for any vaccum leaks, this is most probable. Second, if you remove the spacer does the stumble go away? I dont think adding a spacer would normally have this effect. If no leks are present go up a couple jet sizes and that should do the trick.
Good Luck...Jeff

You might need a little more squirter the intake volume has increased with the addition of the spacer.