The variations of the length of day and the angular velocity of the relative rotation of the Earth's core derived from geomagnetic secular variation were compared from 1650 to 1985. Periods comparable with the well-known 60 to 70-years periods of the fluctuations in the Earth's rotation could be found within the spectrum of the core rotation parameters by lengthening of the previously used time series to epochs before 1900. Additionally, the torques of the geomagnetic core-mantle coupling were computed for this time interval. The results agree with previously derived values for epochs after 1900. The coupling behaves like a homogeneous and isotropic magnetic friction, and the difference between axial and equatorial mechanical torques derived from Earth rotation parameters cannot be explained by magnetic core-mantle coupling.