kempy, nothing unusual about the black soot (smut) when using 5356. It's from the magnesium in the wire that has a lower boiling point than aluminum; some of it it vaporizes out. It's worse on 6061-6063. You can crank up the Ar which will help. Also a little more radical 'push' angle helps too. When I look at the weld that was done (assuming preheat) at the settings I used it looks like too much wire and not enough volts. I'd go up a volt and down 5-10 on the wire and turn off the synergic function, maybe slow down a little and open up the weld. keep cranking down the wire until the edges thin out. That spatter is also something that's gonna happen more with 5356. That you can only go 1/4" before things get funky is not right.

Spool guns have a couple of things about them that suck. I haven't had one that didn't have a 'rhythm' to it. I think it's from several things: the two biggest, I think, is the necessary small small diameter of the drive roll and the short distance the tightly wound wire off a four inch spool travels which doesn't get straightened out in a long conduit as with push-pull. My spool guns run 1/16" smoother than smaller wire. Periodically, I take the toothed drive rolls and soak them in a caustic soda solution that overnight, dissolves the embedded AL. I have one gun that has a 'twitch' that flicks .035 wire at a regular interval. Drives me nuts. So I don't run that wire in it. Though I do most of my serious welding with a suitcase and 20# 12" rolls I do buy 1# spools off FleaBay because I'm cheap. It's surprising how different brands run differently. I've had some small dia. wires come out at such a radical angle, like a claw, that I'd have to go to a smaller tip to help out. This is one of the advantages of the so-called second generation 30A guns - they make 8 different dia. contact tips.