Hello,
at a guess I would say its not the starter motor I would go alone with neil748r as your first port of call. Has the bike been sitting, maybe check the charge your battery is putting out. It takes a fair kick to turn a Duc over.
Don't you just love Duc's and there electrics . . .
let the fun commence.
regards
TopiToo
[Edited on 15-07-1968 by TopiToo]