If the middle nut is free to turn then the dome nut will only turn the studding to tighten it.
As soon as you try to.loosen it, the dome nut will simply undo from the studding.
In order to stop this occurring, the locknut needs to be locked against the dome nut.
The lower locknut will undo but will be tight, you need to undo that in order to adjust the chain tension.
It's not falling off, it's an upgrade opportunity.
First, lock the dome nut with the locknut, so you can turn the adjuster, not just the dome nut.
I'd suggest using permanent locktite to make sure it doesn't undo again.
Put a mark (Tippex works well) on the dome nut so you can see where it was originally.
Get the engine nice and warm, (they always tick a bit when cold) and, with the bike running on a paddock stand so it's nice and level, slacken the lower locknut, but hold a spanner on the dome nut to stop it moving by itself.
Now you can adjust the cct inwards by a flat or two, to hear if the ticking stops. Ideally you want to leave it just at the point where the ticking stops, so wind out until you just hear a faint tick then in half a flat or so until it stops.
Lock everything up nice and tight again.
If, by any chance, the cct starts to weep a little oil after a short ride, then with the engine off, undo the cct a few turns, counting them, clean the thread, add silicone sealer and wind back in to the original setting, then lock it all off tight once more.
It's easy to imagine a ticking noise amongst the general engine noise- and check by pulling the clutch in- if the noise goes away it's just the clutch, and that's normal.
If you're not sure, leave it alone.
Last edited by tony.mon on Sun Feb 28, 2016 8:56 pm, edited 1 time in total.
It's not falling off, it's an upgrade opportunity.