Paco ~
It was good to Zee you again at the meeting last night. According to what I saw on your car, you're using the FIRST style of distributor, with the internal magnetic pickups inside the distributor housing instead of attached to the outside of the body.
You told me you had obtained a resistor and would try mounting it in-line btwn the coil and the tach to see if that has any effect on your tach (it fixed mine). That's the easier job so I would recommend you try that first. You also now have a late 260Z-early 280Z tachometer, which you could install in place of your current one to zee if that solves your problem. That will take a little more work, since you have a dash cap and SOME of them (not all of them) can prevent the tach and speedo from being removed in the normal fashion (pulled straight forward toward the driver). If that's the case with your car, you will have to loosen the whole dashboard enough to remove the tach to the rear. I opted to remove my dash cap, which destroyed it and my dash. They were glued together and big pieces broke when I tried to separate them. I had to buy a replacement dash.
Crawl upside-down under your dash and carefully pull the connector off the back of your tachometer. Compare it to the connector on the back of the "new" (260-280) tach. Pay attention to the pin count and pattern ~ they will have to be a perfect match for it to work. While you're in there, examine both sides of the connector for any signs of rust or green corrosion. A flashlight and a mirror and having three hands will help. A single rusty pin can prevent the tach from working. If you find any corrosion, clean it out with a pipe cleaner soaked in a baking soda/water solution. If you must, you could just stick the whole connector into a paper cup of baking soda/water and let it soak a minute. You can blow it dry with a hairdryer before you plug it back in. It's fun to hang upside-down under your dash for a minute, holding the cup up to the connector and trying to remember which way to move it so it doesn't spill in your face. Don't ask me how I know that
. Some people just spray a burst of corrosion killer up into the connectors, but baking soda is lots cheaper, doesn't remove paint and you don't smell it for a week afterward.
! Usually any tool I'm using slips out of my fingers and hits me in the eye. I was planning on trying the ballast resistor first before trying to swap out the tach. I was looking at Robin's Z and noticed he had a Petronix without a ballast resistor. He didn't mention any problems with his tach so it must be a later version.
