670 belt too long?

Hi,
I can’t get tension on the 670 belt on my K8400. The shafts are perfectly aligned and the printhead moves with near to nill resistance.
The 685 belt is perfectly tensioned but the 670 is so loose it jumps steps even when fully tightened. Any ideas about this.

PARTIALLY SOLVED:
This printer is extremely sensitive on alignment! Loosened all bolts having anything to do with the carriers and then used the 3D printed alignment tool with everything slightly loose and tightening as I went along. The belt is better but not as tight as I’d like it.

I have the same problem. I checked alignment, bearings, rods etc, reseated everything, but still slack on both 670 belts. All other belts fits nicely. The 670’s are marked correct 670xT2.5, but could it be a wrong marking or a bad production?
Any suggestion for a belt tensioner, or do I need to get new belts; as is, printer will never be stabile.

The long belts must not be too tight.

Here is a correct tension when you push lightly on the belt :

If they’re too slack you can scavenge a spring from a clothes pin and use it as a belt tensioner (you’ll have to open it a bit) :

The long belts seems to have the perfect tension.
Great idea with the clothes pin spring, I will try that.
Thanks a lot.

OMG - this is GREAT. . . I’ve replaced the little weights I had on the bottom belt with the clothes pin. . . worked perfectly. . .