The message .com
from "Dan" contains these words:

Can anyone advise where the adjuster is?

I very much doubt there is one. I'd expect it to just have springs that
hook onto each end at fixed points.

What you could do to slow it down a bit is take the door apart and
install some ballast, though of course as it approaches vertical it'll
only give it more momentum.

--
Skipweasel
Pay no attention to that man behind the curtain.