Role of yellow wire in 115v AC Industrial motor
It is a split phase motor, but there is no capacitor in a split phase motor design. CSCR (Capacitor Start - Capacitor Run) or CSIR (Cap start-Induction Run), yes. But a Split Phase is used specifically BECAUSE it doesn't need a capacitor to make it turn the correct direction.

Here is the wiring diagram. You don't touch the yellow wire (or the blue for that matter).
https://www.grainger.com/ec/pdf/6XH45_2.pdf

You can change the direction of the motor, you cannot change the speed.