tachometer gauge is jumpy

[olebiker]

The tach on my xj works great up to 7000 then stops there. Won't move acts like it is stuck. I haven't worried about it but it would be nice to fix it.

[scott0757]

+ or - 100 RPMs is nothing I would even worry about! May tach can be rock solid and then, normally after passing 5000 mark, it's like a windsheild wiper on crack...rapid swings from zero to well into redline. It's a short or an open somewhere and so far I have not found either.

I would think it's in the tach's pick-up circuit because if the coils/stator were intermittent, I'd expect to feel it in the engine's performance, which I do not. I'm one step away from opening the guts of the tach (like in the Tech Tip on needle repair) to check it's internal wiring connections.

I also had my side cover off (with the magneto coil & stator) and all looked well other than some light surface rust on the magneto and some dry crud that was probably flying around in there when the RPMs get high. Didn't appear to be conductive

As I can pretty much cause the wacky-tachy to happen on demand by rapid acceleration, anybody think it may be related to vacuum advance or some other electrical source that could break down or not like the vibes at higher RPMs. Not causeing me riding issues but it's sure a distraction when its dancing about..