... big big difference from my I... mine does it at 14... which is way too soon.
I have GOT to find out why yours (or all II's) shifts at 24.
OK... tighten the belt tension ?
Remove both plastic covers...
... there are 4 bolts that you have to deal with at the rear hub.. two on each side.. one at the top.. one at the bottom.
The bottom ones run in slots... once you loosen them.. you can swing the hub back farther in the slots.. tighter belt.
Crack the top ones loose... they stay where they are.
Now get your face down there at the bottom bolts... and observe where the bols are in the slots.. (there are marks).
Now crack the bottom bolts loose loose.. pull the wheel back a little bit (you can look at the marks to see how much you moved it).. tighten all 4 bolts back up.