I noticed last weekend that the tach in my 914-6 was acting up. After a bit of driving, it would jump around in the 3-5k range. Usually while accelerating, it would stop moving at 3K, then jump to 4-5k, then fall back to 3k. I recall someone telling me decades ago that "tach bounce" was usually the result of failing or bad points.

I hope to poke around this weekend. I was going to start with checking grounds (related or not, my fuel gauge is a little bouncy too) first - coil, battery, then maybe grounds on the back of the tach?

I also need to go through records. It could be now a couple thousand miles since the last time the car had points checked/gapped.

Otherwise, the engine runs nice and smooth. No hiccups or issues in running through the rev range, just the tach not working quite right.

Car is stock 914-6. 2.0L motor. Marelli Distributor.

Thoughts? Things to check?

TIA

I would say swap out the tach first, not to big of a job.

[quote name='Racer' date='May 4 2017, 12:03 PM' post='2482336']
I noticed last weekend that the tach in my 914-6 was acting up. After a bit of driving, it would jump around in the 3-5k range. Usually while accelerating, it would stop moving at 3K, then jump to 4-5k, then fall back to 3k. I recall someone telling me decades ago that "tach bounce" was usually the result of failing or bad points.

I hope to poke around this weekend. I was going to start with checking grounds (related or not, my fuel gauge is a little bouncy too) first - coil, battery, then maybe grounds on the back of the tach?

I also need to go through records. It could be now a couple thousand miles since the last time the car had points checked/gapped.

Otherwise, the engine runs nice and smooth. No hiccups or issues in running through the rev range, just the tach not working quite right.

Car is stock 914-6. 2.0L motor. Marelli Distributor.

Thoughts? Things to check?
Quote

I would be checking power supply and grounds yes,are you running 914/6 relay board if you are the signal convertor may be faulty,you can sort that with a tachadapt from Ashlocktech.com.
I don't run points anymore in my Marelli.
My tach bounces and was way told it would need to be repaired,i have a replacement not yet fitted.

The 914-6 and early 911 tachometers tend to bounce a lot. North Hollywood Speedometer does a dampen upgrade. I had mine done and it is nice and stable now.

You should give them a call.

Get a tach-adapt from Bob Ashlock, $65
One of the things it can do is cure tach bounce.

http://ashlocktech.com/Home.php

How does the car run? My tach bounced like that but also broke up at higher rpm.

Turned out to be the points/distributor. The distributor had too much play in the vacuum retarded plate and the play would cause the points to go out of adjustment

Fitting optically triggered "points" fixed it. My theory is the optics are more tolerant of distributor slop.

Tach now solid, motor pulls strong to redline.

My tach bounce was fixed by fixing the relay board.

After 40+ years, it had a couple of cracked solder joints between the top and bottom.

Said that,read my reply ?,if you get bounce it can't be fixed with any little boxes,if your tach has an issue it is stuffed,PO supplied one with the car and its rubbish.
Yes you can send to 2 vendors that are noted for their work but @ what price,and held @ ransom for their turnaround ..really it is cheaper to buy a working unit or just do good gauges yourself for tidy up not that hard.
Bob and I talked about my tach and he said it it is faulty,I except that,he really knows his stuff.
Bobs unit don't essentially cure bounce they are more there to correct signal from unmatched systems i.e. 1972 tach with 3.0 motor for example,differant ignition systems or conversion to 4/6 and 8cyl motors not a faulty gauge by any margin.

Didn't get around to checking the basics yet, but thanks for some other ideas.

As mentioned, the car seems to run fine. Pulls cleanly from Idle to redline smoothly and crisply. This is a recent issue (ie, this spring) and the tach is original to the car and hasn't behaved this way before.

I have some experience working on these tachometers, as Mark Henry would say "That's what I do"

I bench test these tachs with my own home made signal generator, which will supply a very clean signal. Even with a very clean signal, I have observed that some tachs will bounce more than others. I have yet to see a tachometer that doesn't bounce or have a RPM over-shoot issue. Granted, my frequency sweeps are much more dramatic than just someone revving a engine, but it is clearly noticeable.

These tachometers do not have any dampening, so you could add a tiny drop of CST 10000 in both bearing plates to help with the bounce. Also, both capacitors are certainly shot being that they are approaching 50 years of life. I have rebuilt and replaced nearly all electrical components in this style tachometer, and will still observe a bounce.

There is another work around if all your pre-checks do check out. I have a tach conversion thread in my signature that you could check out. It walks you through on how to modernize your existing tach with a cheap off the shelf modern tachometer sold at your local FLAPS. The newer motor is friction-less and doesn't utilize a hairspring to return the needle back to 0 like your original tachometer has.