Bouncy Tach and Speedo (Especially Tach)

[Longitudinal]

I had my first chance to ride the 1982 GS1100GL for more than five miles, and found that the tach and speedo do not behave quite right. Both tend to bounce or wander a bit, especially the tach; I could actually sometimes observe RPM decreasing momentarily or holding as I accelerated.

Is this sort of behavior normally attributed to cables that need to be replaced or lubricated, or to the instruments?

[cowboyup3371]

Start with lubing the cables first

[Rijko]

Also pay attention to the tach cable when remounting.
Sometimes it gets loose, slipping out of the rev counter temporarily and getting grip again.
Make sure the cable is inserted all the way and the "nuts" are fastened all the way.

Speedo cable wobbling could also mean the cable is bent somewhere or the inner cable is damaged.

[Brendan W]

I had my first chance to ride the 1982 GS1100GL for more than five miles, and found that the tach and speedo do not behave quite right. Both tend to bounce or wander a bit, especially the tach; I could actually sometimes observe RPM decreasing momentarily or holding as I accelerated.

Is this sort of behavior normally attributed to cables that need to be replaced or lubricated, or to the instruments?

If cables are dry or have a strand broken they can drag and behave like torsion springs, storing up energy and then releasing it giving an impulse to the clocks.
I imagine that loose connections could do the same kind of thing if the magnet sees nothing and then contact in a very short time.
That both are acting up seems a bit odd. The only common cause there I can think of is the mounting rubbers or cables not fully seated at either end?
Could the clocks have been off and the cables reattached without checking that both bottom ends were engaged.