Topic: tachometer needle swings wildly

[Six]

I bought and installed an NOS tach which looks great, but does not give me accurate readings.  It does not stay steady above 3K rpm's, instead it swings wildly between 4k and 11K when I gas it, and even during steady throttle riding, it swings about 2k up or down. 

I've checked the cable, and it's not broken or kinked.  Do I just have a crappy tach, or is there something I can do to fix it?

Thanks!

Six - Mpls.

[sradja]

Is the tach for your year and model? Sometimes on ebay people list tachs and speedos as one thing when they are not so.

[Six]

I thought all SOHC CB750 tachs are the same!

[ericstew]

The tachometers on old bikes use magnets to hold the needle in its reading position.
If they are de-magnetized,the needle just swings like yours.
It happens on many vintage bikes either on tacho or speedometers.
There is not much to do as the cost to replace the magnets is close to a new tacho...

[Jonesy]

What condition is your tach cable in? If it's binding, it could be twisting itself up and giving the tach varying input.

[Tower]

I just snipped out my old broken High/Low switch and soldered in a new one  into the left-hand assembly on my 750K3.  The headlight still doesn't work, but now the tachometer swings wildly and erratically. wtf? Mere coincidence?

Took it for a spin with the wild tach.  It seemed to begin to settle down after about 30 miles, but not really.  Doesn't matter whether its at idle, WOT, with switches on/off or in either High/Low position.  Any ideas?

[OldSchool_IsCool]

The tach is 100% mechanical, I think it's a coincidence.  Check that your tach cable isn't kinked.  Maybe unscrew it from the tach and drip some oil down it.  Pull it out of the head and make sure there are no chewed up teeth on the spline gear.

[333]

Lube the cable. Dri-Slide or some other dry lubricant works well.  If that doesn't work, replace the cable.  It's binding somehow, lube might fix it.