Simon; Good info from GP, and general dittos to comments...if you engage OD, the resulting drop in RPMs will bring the cacophony at those cruise speeds down, which is always nice...(so will a carpet kit and sealing all the holes between engine and passenger compartments...even small ones!).
Regarding Tach...I was under the impression that if they work with the Elec Ign fitted, they work (and not intermittently), but you could try reversing sensing polarity at the back of Instrument. They can develop age related issues...see:
https://www.sw-em.com/Smith's%20Tachometer.htm
GP; Was that on the mile wiiiide beach at Rømø? I enjoyed my visit there a few years ago.
Cheers