transform.Rotate is rotating all axes, why?

I have been working on a helicopter and I have the first rotor spinning correctly, however the back rotor I have tried everything and I just can't get it to work.

My objective is to get Rotor2 to ONLY rotate on the X axis, although with what I have so far, all the axes spin. What am I not doing right? Why did Rotor 1 work perfectly on the Y axis, but rotor 2 not on the X axis, hmm?

transform.Rotate (0, spin * -0.01f, 0, Space.Self); // Rotor 1

Rotor2.transform.Rotate (spin * -0.01f, 0, 0, Space.Self); // Rotor 2

 using UnityEngine;
 using System.Collections;
 
 public class NewEngine : MonoBehaviour {
 
     public GameObject Rotor;
     public GameObject Rotor2;
     public float spin = 0;
     public bool EngineOn;
     public Rigidbody Player;
     public AudioSource HeliStart;
     public AudioSource HeliConstant;
     public float HS = 1;
 
 
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
 
 
 
         if (HeliStart.pitch > 1) {
 
             HeliStart.pitch = 1;
         }
 
         if (HeliStart.pitch < -1) {
             
             HeliStart.pitch = -1;
         }
 
         if (Input.GetKeyDown (KeyCode.E)) {
             EngineOn = !EngineOn;
         }
 
 
 
 
 
         if (EngineOn == true) {
             spin += 0.5f;
             transform.Rotate (0, spin * -0.01f, 0, Space.Self);
             Rotor2.transform.Rotate (spin * -0.01f, 0, 0,  Space.Self);
 
             HeliStart.GetComponent<AudioSource> ().enabled = true;
             HeliStart.pitch = 1;
             Player.GetComponent<Rigidbody> ().mass = 1.25f; // Set the GO's mass to 5 via the Rigidbody.
         
             if (!GameObject.Find ("Player").GetComponent <ShipControls> ().FuelConsume) {
                 {
                     GameObject.Find ("Player").GetComponent <ShipControls> ().FuelConsume = true;
                 }
             }