Random motion on only one of the 6 axis

I have set up a particle system with random motion towards and away from the camera
with a z parameter.
But I only want random motion in the positive z direction.


I even tried the negate or reverse algorithm to have random motion only in the positive z
and not the negative z direction.

I can not find a way to do this in Motion 3, has this been addressed in M4?

could you send me the Project file and I can take a look for you to see what you mean.