October 5, 2016: I had a jam in the support feed tube. It would feed for a while and then just stop. It must have been a partial jam, but I cleared out the nozzle once and it didn't really improve. I thought I cleaned it out completely, but perhaps I didn't. I went to clean it again and got it working perfectly again. I removed the entire extruder nozzle assembly, and here are the steps I took with it:
1) Soak in limonene for a couple days. I don't know that this did much. I periodically took it out and heated the block up with a torch, which made some plastic ooze out of the back end, but probably didn't help much.
2) Soak in acetone for a couple days. I don't think this did much either.
3) Ultrasonic clean in acetone. This softened the HIPS enough that I was able to push in an insulated stranded wire of about the 1.75 mm diameter of the filament. I would push it with a pliers close to the inlet, and the wire would kink and I would back it out and start with a new piece. Eventually, I noticed the wire was going in farther and farther. Finally, acetone-soaked HIPS oozed out the nozzle and the whole tube was mostly clear. This happened last time too.
4) Ultrasonic clean in limonene. This was intended to clean out the remnants of the plastic from the tube.
5) Found a little tiny broach that I pushed in the nozzle and spun a bit to clear it out in case anything was left in there. A drill bit probably would have also worked, but I didn't have one small enough. I know there are kits you can buy for this exact purpose, so I may get one of them. I suspect that is all I would have needed to do.
6) I noticed the inlet of the ABS side is flared, but the support side is not. I thought the HIPS might be getting caught on the inlet before it could get into the hot zone and melt and that might be causing the problem. Because of this I *GENTLY* tried to flare the HIPS side with a punch. It maybe flared a tiny bit.
7) I gently sanded any burrs that might be on the inlet side with sanding cord.
After all that, it ran perfectly. I don't know which of these steps were the most useful, but I listed them all in case someone else needs some ideas.