floor pans

How does the wire speed effect being able to get a good bead. I am on 3 1/2 and turned up to 4. Seems better in a few places, but not what I am after.
Any idea of where the wire speed should be set.
Lincoln flux core wire feed
I know I have been told I should have this type of welder or something else, but this is what I have to work with.
Any advice