Hey, Ive got an 87 fc n/a and there is something strange going on, my tachometer jumps up and down, but only if the head lights are on. I'm not really sure what to do about that. I figure i'd ask about it while i have my intake off to reground the ecu, seeing as it may be an easier fix with more room to work with. A less pressing annoyance being that the clock works when it wants to.

 

So, the tach jumps up and down when the car is idling w/the lights on? Could you possibly take a voltage reading from the output wire of the alternator when this happens.

 

It does it both when at idle and while driving, but only when the lights are on

 

It would be good to know if there were voltage fluctuations responsible for your problem. There are numerous grounds you could check. Also, the tach runs off of the trailing coil signal and the coil receives its ground from being bolted to the fender. So, clean up this ground/contact point. Chances are this will not cure your problem, but it takes little effort to do, and won't hurt anything in the process.

Grounding link: http://www.aaroncake.net/RX-7/grounding.htm