Prusa MK2 Extruding bumpy blobs

Hello!
I have run into a strange situation and decided to post here to get some ideas and see if anyone's got into a similar problem.
I've been using an original Prusa i3 MK2 for the past year without problems, but today I have started to notice the extruder extruding big bumpy blobs in some of my prints.
At first I thought this was a one off situation, so I decide to print the same model using the same settings. But there it happened again! In the exact same location, the extruder extruded a lump that is taller than the layer's height.
So I thought there was a problem with the model, so I tried another model - but it happened again. Then I printed that same model, and it had the same problem in the exact spot again.

Here is a picture of the problem (also attached below):


Zoomed in:


I also captured a video of the moment this blob was extruded:
https://www.youtube.com/watch?v=BE4yp6B3Ad0

I've checked for under/over extrusion (measured the amount extruded after sending a command to extract a specific amount, then dividing it out and got an extrusion multiplier of 1.06). And that doesn't seem to be a problem. Also, both my X and Y belts seem to be tight to spec (makes a ping sound).

I'm out of ideas! Anyone else here has encountered this before and can point me in the right direction?

Thanks for reading, and any help will be greatly appreciated!

Edited 1 time(s). Last edit at 05/01/2018 02:54PM by hipsterkevster.

When I had this problem, nothing fixed it until I changed the nozzle.
Try this experiment.
Manually place your head about 20mm above your print surface.
Manually extrude some filament (I do this at 5-10 mm/s - This isn't a retraction, you can't usually burn it out at 50mm/s!
Do the extrude at least 10mm at a shot.
Does the filament come out kind of thin and curl up around the nozzle instead of spewing out to coil up on the bed, no matter how often you do the extrude?
Does it eventually (after many extrudes) "pile up" around the nozzle and finally start to spew forth and coil up on the bed?

If so, your problem may be detritus lodged in your nozzle that moves around and occasionally gets pushed out or into the way. You end up with filament stuck in the nozzle until the blockage moves and then you suddenly blast a blob of super-heated filament out all of the sudden. The solution is to swap nozzles, then "burn out" the gunk in the nozzle and use a strand of wire to push the blockage out of the nozzle. Another way that might clear the gunk (if you don't have a propane torch to burn out the filament) is to heat the nozzle to about 20C less than your extrude temperature and manually shove the filament down into the nozzle until you feel it "squish", then quickly pull it all the way back out of the print head. This sometimes will "grab" the debris and pull it back out with the filament.

Of course, it could be you have a bad spool of filament that has moisture or contamination... I have seen what you are talking about in two printers and this fixed them both. Sometimes the blocked nozzle manifests itself as spidery and weak infill.

Good luck!
DLC