missing half step

Hi there, I hope I am in the right section.


I have rebuilt a old stratasys genisys xs 3d printer and have gotten it to print amazing parts but I found its losing step in the x axis.<br>
If i swap it to the Y axis it works but I found the reason is the steps per mm are to low on the X.<br>
If I increase the steps then it will do a 1/16th step when controlling with the computer or the lcd controller but wont print accurately size then.<br>
Is there a way to control or modify it in the marlin firmware?
define DEFAULT_AXIS_STEPS_PER_UNIT 39.513, 65.546, 272.862, 140.65


I am using a ramps 1.4 board<
You can see on the side of the raygun the rings from missing a step.
Any ideas would be appreciated as this printer is built like a tank and like to get it running perfect.
The dark photo is of the pulley and belt it runs on.