Extruder Problems

I've got a BitsFromBytes Rapman, and I'm working on converting the accursed thing into a Darwin. I've replaced the BFB electronics with the RAMPS 1.4 from gadgets3d, the latest version of Marlin as my firmware, and I'm running KISSlicer as my Gcode generator.

So far, I've got all the systems online, in terms of operation. The cartesian robot works beautifully, the hot end heats and measures like a charm, and the extruder spins and moves the filament. Aye, there's the rub.

I've got it set to 20 RPM in KISSlicer, and the speed limit in Marlin's well above that, but it can't be spinning more than once or twice a minute. It's capable of running the full speed, I've got the destringing retract-extrude enabled and that works at full power. It's just that during normal extrusion, it goes glacially slow regardless of what I've got it set to.

Any ideas?

It depends on what filament size your are using, nozzle size inputed, etc. Also e extruder multiplier can cause it, if set too low. Make sure that you have calibrated your extruder. Make 20-100mm difference on your filament starting at the entrance to the hotend. Run Pronterface or Repetier and tell the extruder to extrude whatever you marked. If the top mark doesn't land on the spot where the first make was then your steps in the firmware are wrong.

I'm running the default settings for the printer itself. Some guy already found all the right values for a BFB extruder, and I'm using those for now. Regardless, minimum RPM is minimum RPM, right? Even cranking it up to 60rpm minimum brought no visible change. I'm guessing it's either a problem with Marlin or Kisslicer, maybe the communication between the two, but I honestly have no idea where to look.

The Gcode prefix I'm using is

G28
M82
M109 S155
M227 P1600 S1600
M101

Can you share your FW that suits the RapMan, i have a friend with a blown up BFB print and now mounted a Melzi board.
Any FW that would work would be appriciated.