This article is from the Ducatis Motorcycles FAQ, by Duke Robillard firstname.lastname@example.org with numerous contributions by others.
(Thanks to David M. Lewis for point out that the key goes between the belt
and the *idler* pully, not the adjusting pulley.)
-- the quick-and-dirty (and surprisingly accurate) method for adjusting
belt tension on two-valvers and four-valvers with belt-driven cams.
Surprisingly, this technique is taught at the Ducati Service Mechanics
school, even though Cagiva supplies a very-expensive tool resembling a fish
scale to complicate this simple task.
Since there's very little actual tension applied to the belts via the valve
gear (remember, this is a desmo, although there are retainer springs), the
trick is to be able to pass a 5mm Allen key between the idler pulley and
the belt. If you can't fit it in there, it's too tight; if the belt is
loose enough to fit anything bigger, it's loose. That's it.
While you're there, you might check the pulley bearings, as they have been
known to go away from time to time. If they rotate roughly, or not at all,
The adjusting process itself isn't particularly critical, but what is
critical is that you don't want the belt to be so loose as to hop a tooth
or flap around, nor so tight that the cylinder and head expansion stresses
the belts fibers and causes it to break. Trust the Church's cumulative
experience -- bad things, bad things happen (to paraphrase Dennis Hopper).
There is an equivalent trick for four-valvers. Find the SHORT run of the
belt between the two pulleys:
this one, not the long one | v ______ (O O) \ / | |