my tach is not accurate - jumps all over the place?  worked fine forever - but this season just choppy and unreliable.   1973 260z - flat top carb model.  car has only 35,000 miles on it

If it is a 260Z it could be a sign that the electronic ignition module is having problems.

Or the tach might just be dying.

How long is forever?

purchased new in 73 - so i guess forever (ends now) - ignition module seems fine.  ho does one get the tach out ?  in fact how can u get anything out of behind the dash covering ?

The tach counts sparks.  The module could be sending extra pulses to the tach.

You're going to need to find a full size computer and download the 260Z Factory Service Manual.  The details of how to remove the tach are in it.  It comes out from the front.  http://www.classiczcars.com/files/category/12-260z/

Your car is a 1974 model.  SteveJ knows the 260Z but he likes a good description of the problem.  Your forum style might not work.  Worth a shot though - @SteveJ

Good luck.  Add your location to your profile.

thanks -- yes my build date was august 1974    SN  -  RLS30-046120

Another possibility, though it may be remotely is the resistor is dying or not making good electrical contact. I say remote because resistors usually fail open or closed, not displaying intermittent failures while dying.

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If you still need some answers to your questions...

There are two screws holding the tach in that are accessible from under the dash and under the top overhang of the tachometer hood. Once these are out, I think you just reach back and push the tack forward and pull it out from the front. You might need to remove the steering wheel to get it out. If you have a dash cap installed, it will likely need to be removed.

Your can then attach lead wires to it. One to the + side and one to the - side of the battery, then one more to the - side of the coil after first passing through a 50 kohm resistor. That will at least tell you if the tach is at fault or if it's a problem with a loose wire or bad resistor. Let me know if you need a resistor. I have extra after setting up some test leads for this very issue.

Jeff