IIRC, it was the EPR (Exhaust Pressure Ratio) Due to the iced probe, the instrument reading told them they were getting takeoff thrust when in fact they were at much less. If they had just looked at the RPMs they might have immediately known something was off with the EPR. Instead, they waddled down the runway, lumbered into the air and stalled.
That may have been compounded by icing on the wings, but the root cause was probe ice and pilot fookupitude.
