Erratic tach, missing and idling poorly

[cegrover]

Hi all: I haven't started digging into this much yet, but searched and didn't find my exact symptoms. Here they are:

1) While driving along and with everything working fine, my tach will all of a sudden jump to much higher numbers and then move erratically.

2) At the same time that the tach act up, I'll notice some rough running - even at speed I can hear the car missing. Althought the engine starts running rough, it's obvious the tach readings are incorrect (jumping very quickly and reading 5000+ RPM when I'm in 4th gear at 50 MPH, for example).

3) If I come to a stop while this is happening, the idle is very rough and generally slow.

4) Here's the fun one: this can happen for a few minutes and then just stop and all will work great. Also, the last time it happened, I turned off the ignition, restarted and all was well (rebooted...).

Details: Stock 1975 280Z with 186K miles which has bene sitting for a few years up until the last two months.

My initial thoughts: Definitely electrical and something that can impact both the tach and the ignition. I think this must be a connection-type problem at the ignition control unit (ECU in modern terms, I suppose) or something else electrically 'global' like the voltage regulator or alternator.

My plan: Check all obvious connections to these three major components, then go from there if I don't find something loose.

Any thoughts or experience related to this? Does the tach get its reading via the control module? My car doesn't live at home, so I haven't had a chance to chase this down since it happend on Sunday; but I wanted to bounce this off the group...

[Darrel]

check for any side to side movement of the rotor shaft inside the distributer.

[cegrover]

Update...six years later. I never did get this solved with the stock engine, but it recurred with the stroker installed. It was traced to the ignition box in the passenger side footwell. A good condition replacement unit completely solved this problem.

Yes, it's an old, short thread, but I wanted to close the loop in case anyone else had similar problems.

[SteveJ]

You can also use an HEI to replace the stock box.

[GeoJoe]

Now that's someone dedicated to following up on a post!