tach needle jumps and skips. need some ideas

[FetusGoulash]

this is a little hard to explain so i will try my best.

when driving at a steady speed, for example 55km/h, with the revs at a constant rpm, say 3200 rpm, i see the needle fluctuating once and a while. It will be holding a constant rpm then for a split second drop and spike within about 500 rpm. this only happens when i am barely on the throttle (cruising in traffic for example). If it happens under hard throttle i don't notice because the needle is moving.

Now when this happens i don't feel or hear the engine sputter or pop like the needle should be indicating. It makes me think it might be something in the cluster, but not sure about that.
I also checked the distributor for play as many people have mentioned that is a common problem. i don't know what play i should be looking for though. with the cap on, no part of the distributor has any play. if i remove the cap and pull on the rotor it will move very slightly up and down. Is this normal or does that indicate a problem with the dist? I think a distributor issue could also lead to the weird tach needle jumping?

any ideas would be great
thanks

[alor]

With my car I would experience a periodic blip in the tach needle with no apparent miss, but this progressed over time into a noticeable miss which was synchronized with the needle jumps.

I don't think that a small amount of up and down movement in the rotor is abnormal, but if you look into the cap and find that the tracks left by the rotor are just at the tips of the four contacts, then the play might be causing the impulse to be interrupted.

When you set the timing with a light, is the mark on the flywheel steady and does it mover smoothly with an increase in engine speed?

Alan

[Jaxx_]

I am very inexperienced with E30s, but it might be worth me mentioning that when my battery voltage was low (and my alternator was going bad) I saw the needle jumping a lot. After a fixed alternator, the issue was gone. I doubt this is causing your problem though.

[achillies]

tach

I had all sorts of issues related to my tach. It wouldnt rev above 4300 rpm, skipped, wouldnt settle on an idle speed. I had all sorts of suggestions and I tried them all. Finally I replaced the tach in the cluster and it all was better. It turned out the car did rev past 4300 the tach just didnt show it, the idle did settle it just wasnt being displayed, and there were no real skips just some electrical whatever in the tach. So you might want to attach an external tach to start with or just find a good used one.

[a3ternus]

I think its a cluster issue,not motor related.
N.