Ive noticed that the tac needle for my 75 R90S starts to jump heaps once I get to around 3000 revs. When I say the needle jumps im talking from one end of the tac to the other. Bumps on the road make it worse but even on flat surfaces it does the same thing. Could it be just a worn out speedo cable or something else?
I would definitely change the tachometer cable. I'd also not ride the bike anymore or at least disconnect the cable until this is fixed. Eventually the tach side will be damaged and your repair bill jumps quite a bit.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.
I would use a graphite lubricant. Oils and greases attract dust and gum up. Just like on key locks. Oils and greases are great for control cables (clutch, brake, throttle) but not for speedo/tach cables that rotate at fairly high rates.
Pull the inner cable out and shoot the graphite in then reinstall the inner.
No need to lube these cables. If that is the cause of the needle fluctuation then it is either kinked or has a broken wire strand and needs to be replaced. If it's not the cable then there's a problem in the tach.
Since "bumps on the road make it worse" it is very unlikely that the cable is the issue.