Maybe this will help:

My Quick Tips Playlist

You have the speed of the second and third keyframes set to zero. If you want a descending loop, you'll need to select the keyframes one at a time and move them up the scale about 2/3 of the way, and then adjust the inside parts of the bezier handles so that the speed drops to near zero about halfway between the keyframes.  You may get some unexpected timing in your animation. You cannot set the speed below zero.

I am not sure what kind of timing you are trying to achieve. I hope this helps.