Aprilia Pegaso 650 1993-4
There's a small rubber blanking piece which you remove
from the right side of the right carburettor - connect your vacuum tube to this outlet.
It's best done with a pair of long-nosed-pliers as everything's so cramped in that
area you can't get your fingers near it. I managed to
fit mine (to my '94 model) without removing anything
more than the seat.

http://autos.groups.yahoo.com/group/aprilia-pegaso/message/3169