If your getting strong kickbacks it indicates the ignition is still advanced a bit too far when it isn't running.
Like Creaky mentioned, this could be caused by weak advance return springs in the centrifugal advance.
You need to test the springs.
A quick way to test these springs is to gain access to the points cam and possibly the centrifugal flyweights.
Moving the flyweights outward will advance the cam. Usually, you can just rotate the cam in the advanced direction with your fingers and the flyweights will move out a bit.
Anyway, advance the points cam a bit (against the spring pressure) and then let go of it.
After the springs have done their job, try to move the points cam further in the "retard" direction.
If you can do this, the springs are either weak or stretched and they should be replaced.
After doing this, if the springs moved the points cam to the fully retarded position (like they should do), the points timing needs to be adjusted further by moving the plate.