the problem would be in the centrifugal speed governor at the back of the unit.
It is not intuitively obvious the way it functions so it maybe problematic to fix it without someone who understands it being able to see it. I have fixed these before but I would need to have hands on to do it again.
If you can remove the cover and observe it operating (or not) and are mechanically minded, you may be able to discern what is wrong. Be aware that device is handling the full AC voltage to the motor - LOOK, DON’T TOUCH when it is plugged in.
Newer models may have an electronic device instead of the mechanical centrifugal governor.
Basically, the lever sets the position of the contacts on the governor. As the unit speeds up it rotates the governor weights which move the contacts apart. When they reach the limit, the machine shuts off, as it slows down the contacts are made and it speeds up. It is called a “bang-bang” system. It is either on or off.
If it is always on at low speed, I would guess that the speed lever is not moving the governor mechanism, but the governor is working - otherwise it would be running at high speed, always on.
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