C152 - Propeller goes off on Master only (with keys out)

[atceng]

Starter jam?

Mostly ignition switches fail to make contact rather than develop shorts,which are more common in wiring faults.
The starter motor pinion could be jammed in the engaged position.
.On starting the starter motor solenoid firstly pushes the starter pinion into engagement with the start ring gear and at the end of the travel of the pinion a switch inside the solenoid powers the starter motor.
If the motor pinion jams in engagement the solenoid switch is permanently on and thus power to the motor is supplied as soon as made available by the master switch the motor will then turn the engine and prop.
The ignition switch in L R and BOTH positions only enables the selected magnetos and has no effect on spark timing.
However the above is pure conjecture and if it isnt the jammed pinion you need to make electrical tests to track down the fault.