RD400 tach accuracy

[RD400GUY]

Still getting to know my new-to-me '78 RD400. Can't ride much due to our gawdawful rainy & cold PNW weather, but I'm wondering why the tach bounces around so much at "idle" speeds. The idle is probably set too high, btw, around  1800 -2500, but the tach bounces around a lot at those speeds only, like 1000 RPM deflection. Funny thing is, I just don't feel the RPMs are actually changing that much. Anybody else experienced this?

[rodneya]

Drive cable may be kinked or binding inside the housing, or just old.