When I have issues with starting an arc (usually from a less-than-spotless electrode), I find that touching the feed wire on the metal near the location of where I aim the electrode will start the arc. Static electricity is what makes this work, I think.
400 PSI is plenty of argon to weld with. What do you have the regulator set at? 15 to 18 PSI is what I use for aluminum. Less than 15 and it wont start arcing. More than 18 just wastes argon.
With a unit with as much age on it as yours may have a leak somewhere in the argon delivery (fittings, hose). When this happens you will be able to make all of the heat and noise of welding, but nothing will stick together (especially with aluminum).
Carl
