unwanted rotation in y axis

Hi. My motor makes wheelie on rear wheel on pressing shift. I'm trying to make it another wheelie in the opposite direction on pressing ctrl. I used the Slerp function. It works, however the motor also rotates in y axis. I get this unwanted rotation when I press either shift of ctrl to make wheelie. Motor rotates in y axis to face world z direction. I tried freezing this rotation through rigidbody but it doesn't work, and use different functions with quaternions and vector3 but with no results. I tried both in transform.rotation and transform.localRotation. Can sb tell what is the cause of this rotation?

this is my code:

In FixedUpdate

         deltaRotation1 = Quaternion.Euler(-Wheelie, 0, flipRotate - transform.localEulerAngles.z + (MotorRotation));
         deltaRotation3 = Quaternion.Euler(Wheelie, 0, flipRotate - transform.localEulerAngles.z + (MotorRotation));

         deltaRotation2 = Quaternion.Euler(0, 0, flipRotate - transform.localEulerAngles.z);

         myRigidbody.MoveRotation(myRigidbody.rotation * deltaRotation2);

The other part in Update:

            if(shift)
             {
        
                transform.rotation  = Quaternion.Slerp(transform.rotation, deltaRotation1, Time.deltaTime * 20); 
             }

             if (ctrl)
             {
                transform.localRotation  = Quaternion.Slerp(transform.rotation, deltaRotation3, Time.deltaTime * 100);
      
             }