Hello Dennis, you have supplied some of the information that is necessary to make a judgement call on whether you can weld aluminum with your set-up or not, however there are quite a few other variables to consider as well. Thickness and size of the item that you are trying to repair or weld can be a large factor. The Hobart 140 wire machine that you say that you are planning on using is a very limited machine both in amperage/voltage output and also in duty cycle. Aluminum welding itself is a challenge with many much better suited machines let alone the one that you are considering using. If you are dead set on attempting to weld with your current set-up then you might want to consider a few things: use preheat prior to trying to make a weld, blackening the aluminum with only the acetylene on and then switching the torch to a neutral flame and running the torch over the area until the carbon has burned off will provide approximately the correct preheat, you might also consider brushing with a stainless steel brush after preheating to insure cleanliness and remove oxides, to go a good job of welding aluminum requires it to be as clean as possible and free of any and all types of foreign materials, if possible use a 5356 filler wire, this wire is stiffer than 4043 and will be less likely to give you feeding problems, if you end up with a cracking problem in welds you may have to go back to the 4043, the argon shielding gas will work, but a helium/argon shielding gas will yield more usable heat from your welder, you can experiment with a drag/push angle on the gun to come up with an acceptable bead profile, generally a drag will yield better penetration and a more convex bead shape and pushing will yield less penetration and a flatter bead shape and finally if your gun has a steel liner in it you may want to see about getting a nylon one to help with making the wire feed better, also, in any case make sure to keep the liner as straight as possible at all times. As a final note Dennis do not use an oversize contact tip in the gun, use the size that matches the wire diameter, oversizing the tip can cause arc stability issues and cause premature failure of the tip. Good luck, Allan
You will have much better luck with a larger diameter wire.
.035 or .045 will allow a good deal more heat from the welding machine, possibly reducing your need for preheat.
On the subject of preheat, kepp it below 350F if you want the mechanical properties available from 6061.
Good Luck
brande