You have to take the DEC cover off, straighten the belt and then tighten it so it has no more than 3 mm slack when you press it down in the middle. Having the proper tension and it running straight is key.
Nonetheless, this remains finicky. For instance, I can now get outstanding DEC guiding with an RMS of 0.5" early in the evening, only to see it fall apart and disintegrate to >1" after the meridian flip. That shows the mechanical limitations of the mount.
You will have to get a feel for how much tension the belt should have and whether the axle is running tight, but without excessive resistance, etc. A real problem IMO is that the DEC motor is drawing about 40% less power than the RA motor, which results in it losing steps whenever there is too much resistance.
Also, make sure that the mount is perfectly balance in DEC and that there is no cable drag. With those weak motor, you have to strive to eliminate any potential mechanical interference.
Hope that helps,
Jo

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