So the problem is when the transistor is powered(it gets pretty hot) and the output is the motor is only 1.54v??? the setup is down below(i don't have the button or the diode). Can anyone help me? I am guessing it's probably the transistor's resistance.
The solution is to use the best parts for the job and don’t omit parts that are used in a circuit. You absolutely need the diode across the motor, without it things go poof as in blow up, fail. You’re also missing a current limiting resistor between the Arduino output and the transistor base. That’s another no-no which can wipe out your Arduino.
While you could probably make that circuit work with the Darlingtion transistor, you’ll not get full battery voltage to the motor. You’ll typically loose 1.2 volts out the 6 volts available with a Darlington. Once upon a time, they had their place in electronics but that era passed at least twenty years ago. Today, the far better part would be a logic level N channel mosfet, something like this:
