The problem with single phase is that there is no relative rotation of the magnetic fields. Once ALREADY spinning, the repulsion between the magnetic fields of the stator and rotor is against a moving mass, so it KEEPS it moving in that direction. But at a stand still, it just vibrates back and forth; it doesn't know which direction to rotate yet. The start winding is there to create that initial nudge to get it going in the direction you want it to spin, it does this by creating a virtual phase shift between the two sets of windings, using the capacitor (in the case of a cap-start motor), or a DIFFERENT capacitor (in the case of a cap-start / cap-run). That only needs to ESTABLISH that shift for a second to make the motor BEGIN spinning, then it becomes redundant. So the components for the Start winding are typically smaller than those of the main run winding since it is not really tasked with running the full load.
Bypassing the start winding sets up that "no rotation" situation and the motor does not spin. Without the rotor bars spinning through the magnetic fields of the stator providing mutual inductance to create impedance in the circuit, the windings look like a short circuit and blow the fuses or trip the breaker (or burn up).
If the motor was already spinning, it would work actually. the problem is, and especially with well pumps, if the motor is already spinning the WRONG direction, it will CONTINUE spinning in the wrong direction!
