Long time lurker - first post. :P
I have a Sunbeam Cornelius popper that I have modded in stages. I have installed a chimney, drilled holes on the plastic outer case and base, enlarged the vents in the roasting chamber. These have worked well, especially enlarging the vents, to the extent that my roasts have extended from 4.5 minutes to 12 minutes (80 grams).
My last modification was to take the upper half of the plastic case off entirely, so the roasting chamber just sticks out from the base (without any plastic surrounds to insulate it). When I tried to roast like this, I didnt even get to first crack after 20 minutes!
Does the function of the thermostat rely on there being an upper plastic case? Or have I just reduced the heat retention of the entire device so much that it wont get to roasting temperature?
BTW: I replaced the upper half after 2 fail roasts, and the Cornelius went back to stable 12 minute roasts. I made sure that mod was reversible as it is a large step!
Im not familiar with that model, but most poppers have a thermostat as a safety device, not for temperature control (different to your oven or iron).
My guess is the reduced insulation. You could try removing some of the case - although that might not be so reversible!