The bolts that hold the right side aluminum fender on my trailer pulled through the bolt holes. I tried to re-attach it multiple times with no success. I figured, no problem...I have a mig welder & will re-weld the fender. Picked up some aluminum welding wire from Home Depot and went to work. Working with a Hobart Handler 140. I was able to get enough weld on to do the job, but the gun kept jamming. The wire would bind where it feeds into the line to the gun where it "pushes" the wire. It took about 10 jams/clearing of jams to get the job done. Any suggestions for aluminum welding with this setup?

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My lincoln would do the same, if I kept the torch stretched out so the hose was straight it worked much better. Solved the problem with a spool gun.

You need a spool gun to MIG weld aluminum. The spool is held in the gun and not in the machine.

Spool gun.
MIG welding aluminium is OK for structural, but would never be good enough for say an oil tank or head repair, you need TIG for that.
You need a good HD TIG for head repairs.

Mike is right. I even got a teflon liner for my MIG and it's better, but spool gun is the only way to go on that soft aluminum wire.

I used a spool gun on my Lincoln, and got the Lincoln because the spool gun was reasonably priced. I used it to fabricate my intake plenum on the trannywagon. It takes some learning and my welds did improve, but were still rather ugly. You will also need argon, not mix gas.