Originally Posted by
Lotz-A-Landies
Dust, lubrication, kinks and breaks.
speedo cables are supposed to be regularly lubricated with light machine oil. You can almost guarantee that a S1 speedo hasn't had any maintenance for decades. Ideally you should take the cable out, withdraw the inner from the outer and wash both in petrol. Inspect the inner and outer for kinks, if kinks present or fractures in the inner present you should replace the cable, If all O.K. lubricate the entire length with sewing machine oil insert the inner back into the outer and re-install.
you may also need o lubricate the back of the speedo where the cable enters.
If this doesn't solve the bounce the speedo itself needs a service by an instrument fitter.