Sounds like a PSC Motor (P
Without the Blower on the shaft, the Motor will spin at high speed - regardless of the Speed Selection Lead used.
This is because the Motor draws input True Power (Wattage), according to the amount of Work required to do.
Simply, this means that if the Motor is connected through the "High" Speed lead, it will attempt to develope the highest amount of Work, by spinning the Blower at whatever RPMs that would result in the maximum rated Horsepower + speed of the Motor.
In other words, it would move a volume of air, which would require "X" amount of Horsepower, and this HP would be drawn from the Power Supply, in the form of Wattage, within a "Package" of Line Volt-Amps.
For any "Low Speed Setting", the input Voltage is reduced via an Autotransformer, wound into the Motor's Stator Assembly.
Now the level of input Watts / VA's are restricted by the Lower Voltage applied to the Motor's Stator, and as a result, the output Horsepower is reduced - which ends up slowing down the rotation of the _Fixed Load_; in this case is the Blower.
So, in more simpler terms:
The Blower's Speed is reduced because the amount of True Power which is drawn from the Power Supply, is limited.
The Blower will spin up to the speed which moves a given volume of air per the drawn input True Power (Wattage), which is carried to the Motor in the "Complete" Apparent Power "Package" (Apparent Power = Volt-Amps).
The output "Horsepower" developed by the Motor is transposed to the air, and is equal to the input True Power (Wattage) drawn from the supply (plus losses and other stuff!)
Simple, isn't it
You should cap off the unused Speed Leads, so they do not end up becoming an active circuit of their own.
No need to include them in any connections - just cap them so they do not end up drawing current by means of a Ground Fault, or even between themselves.
These leads are taps off the Autotransformer, so each lead is an active circuit by its self.
If you only want to use - say the Highest Speed Lead on this Motor, then cap off the remaining leads, and connect the Ungrounded Branch Circuit Conductor to the "High Speed" lead.
Capacitor leads are not connected to the Motor's "Input Leads" - they only connect to the Capacitor.
The remaining Motor Lead should be Terminated to the Circuit's Grounded Conductor (if used on L-N 2 Wire Circuit), or the Branch Circuit's other Ungrounded Conductor (if using an L-L 2 Wire Circuit).
Lastly, the remaining speed leads should be capped off separately - not all connected together.
If you have ever worked on a "Multi-Tap" Ballast, the same thing is done: cap off unused leads individually.