I agree, below is the magnetic field of a normal magnet, you can see that each end of the magnet the field is pushed out rather than together like in the middle, what maybe happening is with a stock magnet being longer than the hall sensor usually works with at each end of the magnet it is picking up a trigger signal, it would be intresting to see what rpm on the laptop aswell as a tacho around the sparkplug wire displays at high rpm
Briggster is using the GY6 pickup, but apparently the Ecotrons ECU will have trouble dealing with N/S signals. This is from Ecotrons installation documentation and may shed some light?:
"For some engines, such as Honda GX160, GX390, or similar, the stock magnet is North polarity. And the some engines, such as Gx35, the stock magnet is South polarity.
Our Hall Effect Sensor is by default South-Pole Magnet trigged.
If you install this Hall sensor to an engine which has a North-pole magnet, you will have 2 pulses per revolution, one at the leading edge of the magnet; one is at the trailing edge. This will confuse the ECU and you may not even able to start the engine. You may start the engine, sometime, but the running may be unstable or stall at high RPM.
To fix this, we can provide a North Pole Hall effect sensor as the pick-up sensor, so you don’t need install an extra South Pole magnet. But you must tell us before the shipping of the EFI kit. So we can replace the S-pole Hall sensor with a N-pole sensor."