RepRapPro Mendel + E3D V6 issues

Hello all!

I have recently been given access to a RepRapPro Mendel fitted with Melzi electronics. The stock extruder was replaced with an E3D V6 bowden extruder. I cannot get it the machine to extrude well at all, and I am personally at a loss for causes of the problems. I've never seen the machine in a working state so I'm finding it hard to figure out the root cause.

Here's the sequence of events if I try and print an object:

- Print begins, and the printer attempts to print the object. The actual movement of all axes seems to work fine however the printer does not extrude enough material, and the nozzle drags around most of the PLA it has extruded.
- Eventually the extruder drive will start to skip, unable to extrude or retract any more filament.

If I then take apart the extruder head, a small amount of plastic has solidified in the microscopic gap between the PTFE tubing and the top of the heatbreak, preventing the filament from being able to move and hence the extruder skips. No matter how well I try and fit the tubing this seems to happen, and I have replaced it multiple times. I have also noticed that if left at temperature after attempting to print, the machine will very slowly ooze PLA.
I have also attempted to change the voltage to the extruder, but it seems the slipping is related to the jam rather than not enough torque.


Hardware:

- Extruder - E3D V6 1.75mm bowden 0.4mm nozzle
- Rest of printer as stock

Software:
- RepRapPro's own version of Marlin, which can be found here.
The thermistor in the extruder head has been replaced as the original broke. I have read a lot about thermistor look up tables but the breed of marlin I am using does not seem to incorporate this functionality, instead relying just on a b value which has been changed in the firmware.

- RepRapPro's version of pronterface, found here.


If anyone could offer any advice I would appreciate it greatly. I can give further details on configuration as requested. Thank you.

Three tips:

Use a new razorblade to cut the end of the PTFE tubing as flat as possible. Any angle on the cut ruins the seal between the heatbreak and the PTFE tube.

When you first insert the PTFE tubing into the hotend, push it all the way in to it's seated position, pull it back out to engage the black tubing lock, hold the tubing lock in it's pulled out "locked" state, push the PTFE tube back int the 1-2mm that was needed to seat the tubing lock.

Do not leave the hot-end heated while the printer is not running and make sure you are cooling the hot end heatsink will a full RPM fan.