AFAIAA, an extrusion factor of 90 means that you want to extrude at 90 times the standard rate. I have always used a real number rather than a percentage in that field - e.g. to have a 90% extrusion factor, I would put the figure 0.9 in that field. It's possible that Slic3r automatically interprets high numbers as being percentages, or will accept a "%" character as a suffix, I've not tried, but the fact that 90 gave a massive over-extrusion would indicate that it was interpreted as a whole number rather that a percentage.
The effect of using 86 is puzzling. Maybe it caused an arithmetic overflow in Slic3r, or maybe your initial try with 90 cause the extruder to skip so much it acted like a file on the filament and clogged your extruder so that afterwards it was slipping instead of gripping the filament (in which case it will need to be dismantled and cleaned with a wire brush or similar).
Dave
