I take it you are actually referring to the Rev counter and not the Speedo - if so I had similar fluctuations n
ot related to power output or speed. This turned out to be the pick up (Faston) on the Ignition amplifier needing a bit of tension reapplying to the contacts.
Mine is on the LHD inner wing attached to the ignition coil just aft of the radiator and chassis oiler.
![[Linked Image]](https://tm-img.com/images/2025/02/17/11A4D27C-20A2-49F4-A5A2-23EC8098DE62.jpeg)