See, Idon't actually know if it's the tacho mechanism or the cable or an electrics fault. What I do know is...
1. Since a few months back the tacho bounces a lot at idle, but then seems to settle more at 3000-4000rpm. This to me suggested a weakening element where the cable and clock mate up. At lower revs the two mating parts, if worn, can slip and miss, making the clock bounce. At higher revs and therefore a greater rotational speed it gets some inertia so the bounce becomes less pronounced. Conclusion1: It's a clock or cable mechanical wear related problem.
2. Today, by pure chance, I happened to notice that even at higher revs, if I indicate left or right, the needle bounces in a much more pronounced way, even at 3000-4000rpm. Switch the indicators on... off it goes. Switch them off... it settles again. So this reminded me of a chat I was either in, or that I read on here where the role ofthe electrical circuit was discussed in relation to the tacho... in that it then suggests it's an electrical signal fault. In that chat there was talk of some kind of control doo-dad somewhere that is responsible for a number of things including the ability of the rev counter to relay an accurate reading. Jim just referred to it as a monostable, but I have no idea what one looks like or where it is,and google is throwing up absolutely no links to follow, or images of one, to help me identify it.
Also, it's gone back to feeling a bit grotty and weak when I tried to start it when warm/hot again today. Not as bad as before, and it generally started first time but it took a bit longer to kick in to action and was certainly weaker at starting once warm. That leads me back to the mythical eureka bypass wire, which I tried to identify last time but was wrong... (or it was inconclusive as to whether the thing I showed on here was the eureka wire or something else).
That absolute lack of any ability on my part to discern between any given wire and any given electrical component really tells me I need to find a professional car electrician... otherwise I'm just going to be shooting in the dark and will spend weeks fiddling about and making lawdy-knows-what-else stop working in the meantime. I already seem to have lost my headlights with even a tentative fiddle today.
The problem with finding a 'general' car electrician...even one that claims to specialise in classic cars, is that I'll pay a small fortune and whilst s/he might be able to get my headlights working again, s/he'll probably have never heard of a eureka wire, and will just confidently tell me I don't need one then hand me a bill, leaving me back where I started, but possible with some headlights once more.
So I just need to spend a lot of time searching out a likely candidate local to me who I can give the car to and be confident they know what all this stuff is and does, and can say conclusively whether my tacho is electrics related, even whether my dead speedo is electrics related, and give it a really proper looking at to determine what, if anything is wrong, or what, if anything, is right.
I just really really REALLY really really hate electrics. I honestly can't even make any sense out of the advice offered so far. It's like handing me a bucket of wet spaghetti and telling me to rescue the spaghetti noodle called Steven without snapping it. All I see is a big bucket of spaghetti, wrapped around numerous meatballs, and my brain short circuits.