Plastic not being extruded properly.

I'm a relatively new reprapper and I've been printing all kinds of things recently. Only now it seems I've hit a snag. I'm printing with ultimachine green PLA using a j-head (IV- hotend and it seems like it's clogged since it's not extruding properly anymore. I can raise the nozzle and extrude in pronterface no problem, but when I try to print an object it eventually stops printing and starts grinding plastic into the hobbed bolt. The part that it did print is very brittle and easy to break indicating that it's been "missing" all along. I have my idler tension set as high as it'll go as I've always done, and I'm using the same PLA spool that I started with. I've tried increasing the extruder temp to 190 (was using 185) and it printed a bit longer before messing up, but it still messed up. I've been printing at 185 since the beginning and never had a problem before (and I'm using the same spool of plastic), so why now? The first layer prints perfectly (in fact, the "skirt" is even perfect), but then it seems like it has trouble printing the higher layers and I can hear the hobbed bolt skip. When I raise the Z axis after the failed print, it seems like the nozzle is clogged (thinner discharge), but I can extrude manually in pronterface and it seems to clear up. I'm asking anyone who has more experience if they have any ideas as to what may be causing this. Maybe it's the object I'm printing (lots of perimeter, not much infill)? Maybe my nozzle is clogged? If so, what can I use to unclog it?

Do you have a fan pointing at the upper part of the J-Head? Not at the object being printed, but at the hot end itself?

It is a known issue when printing PLA with the J-Head hot ends. If you do not cool the top part of the hot end, then the PLA can mushroom inside and eventually cause a blockage. I say cool, but all you really need to do it prevent it from getting hot. This part of the hot end only gets hot from convection from the bottom. All you need is a small fan to break up this convection.

Yes, I've been using a fan since the beginning. I even printed a new x-carriage that has a dual fan mount (one to cool the PEEK, and one to cool the print) although I'm only using the one that cools the PEEK. In the beginning I ghetto mounted a fan using long bolts and zip ties to the belt clamps.

I just cleaned my hobbed bolt and removed/reloaded the filament. This time, I tried hand-feeding it and boy what a shock as to how much pressure is required to force the melted filament through the nozzle. I can get it to come out the nozzle but only by putting a lot of pressure on it. I decided to try using the motor to feed it thinking maybe I underestimated the amount of pressure required and the motor had no trouble pushing it through. I ran a good bit of filament through to hopefully clear the nozzle and I measured the diameter of the extruded filament at 0.48mm. I'm guessing that's good for a 0.5mm nozzle, but what do I know?

I wish I knew why it keeps jamming up on me when the print starts off good (and the skirt width measures what it should and is consistant). I'm using the new Slic3r (0.9.9), so maybe something is set wrong since I haven't done many prints with it yet. If this wasn't a hobby, I'd have thrown it out a window by now.

It shouldn't require that much force to go through. I use a .35mm nozzle and it doesn't take much force to push plastic through when it's clean. I had similar problems when I had a clog in my nozzle, took me a while to get it cleared. I used a wire which I pulled from a wire brush to clean out the nozzle while it was hot. I don't know what the specific cause was, but eventually I got it out of there and now it prints great.
One other issue I ran into is that as my extruder wore, the filament was wandering out of the toothed part of the hobbed bolt, and this would cause the extruder to just run without pushing any filament. I had to grind wider teeth with a dremel tool and that solved the problem there.

I finally got it to print the part I needed. After cleaning the hobbed bolt and manually forcing filament through the nozzle, adjusting Slic3r settings (fan speed 100% instead of 90%), etc. I actually think my real problem was the tension on my filament spool. I'm nearing the end of the spool and I'm thinking it was wrapped too tight and the angle I was feeding at was eventually causing the filament to slip off to the side of the hobbed bolt and it wasn't able to pull it off the spool. I lowered the temp back to 185 (WTF was I thinking raising it would do any good?) and the print went through without a hitch (not a single skip noise, and no plastic jammed in the hobbed bolt). Now if I could only get rid of the stringies.

Retraction gets rid of the stringies.

I have my retraction settings at 1mm, 35 mm/s using a j-head and I've confirmed that my firmware isn't limiting the retraction speed (or at least I think). What I think may be the problem is when I was putting my extruder together, I noticed there was a little bit of play in the gears. I got the motor as close as I possibly could (even dremmeled the mounting holes a bit to nudge it further), but there is still a little bit of play. I'm thinking I'll try to compensate for it by using a higher retraction length and setting extra length on restart in Slic3r. I haven't started printing ABS yet or I'd try printing a new set of gears (maybe slightly increase the diameter of the small one).

2mm @ 30 definitely works best for me for PLA, seems also to work nicely for ABS. I get goopy edges with anything less.