Detroit RepRap Builders
I have a similar issue with the BfB extruder, very slow extrusion. Checking out the alternatives, the heater and nozzle seem to be similar to other designs, and with additional force it seems to extrude OK. The obvious difference seems to be the drive mechanism.
There's two main possibilites : the screw drive and the heater resistance.
My guess is the screw thread can't generate enough force to push the extrusion through and slips. Once it slips, it chews up the filament and can't get going again.
Give that Nophead's produced some useful figures for the extruder nozzles and forces produced by the pinch wheel design, I'll see if I can measure the output and identify the problem side.