Clean, clean, and reclean.
Use a U-groove with about a 1/16 to 3/32 inch root face, no root opening, no root purge. Make sure you are in the spray mode, don't try to use low volts/low amps. Use upward progression, and point the nozzle upward slightly. Move fast, but steady with no side to side weave. Stringer only.
Practice and more practice. The contact to work distance is important. If you hear a crackle sound, stop and clean up your crater. You will most likely detect some porosity where the crackling was heard. You want to hear the steady "whoosh" of spray transfer to get the best fusion, joint penetration, and least porosity.
Check other threads for details on cleaning the aluminum. It has been discussed several times.
DO NOT PREHEAT the aluminum!
Best regards - Al
What type of joint? Is it a groove? CJP? Backing? What diameter/wall thickness is the pipe? What voltage, wire feed and gas settings are you starting with, and have you adjusted them within the allowance of your WPS? Is this a PQR or a Performance qualification?
Aluminum is a tricky critter, especially when it comes to thin materials; these are better off being welded with GTAW, in my opinion, because you have much better control over heat input than with GMAW. GMAW with 100% argon is probably your best bet here, with the 4043 wire. I would guess you're using 4043 for a reason? If you are making a CJP with no metallic backing, you're going to have a tough time, unless your pipe is something with a heavy wall. 4043 doesn't like to run too cool, so your welds look better if you run them hotter; hotter or cooler in this case is wholly dependent on what settings you're starting off with. For example, a 3/16" plate CJP v-groove, 1/16" root opening, no backing might require 22v, 180ipm and 30cfh. A tube might require less voltage, since the heat will not be given the same chance to dissipate in the circular weld. However, in this case you might consider a preheat, again if it's within the allowance of the WPS.
While I'm at it, I may as well remind you to clean, clean, clean the base metal. When you think it's clean, clean it again! Good luck!
EDIT: right as I post this, I see Al said not to preheat the aluminum. I will amplify my reasons for preheat. If you're using lower voltages, the first 'x' number of inches of weld (depending on material thickness and voltage) will have a tendency to resist proper fusion. A light preheat will assist in bringing the material up to a temperature which is more conducive to welding. By light I mean no more than 150 deg. F. It's not necessary, but if done right can help out a lot.