What causes the bouncy tach that I see in most of the cars? Mine bounces around pretty good, especially on a down shift.

Is there a way to fix it, or do we just have to live with it as an old design?

You should do a search. A bouncy tach is an indication that there is a problem with your ignition system; dwell, gap, timing, etc...

Also i my experience the tach in our cars are twitchy, some have used a capacitor to dampen the effect, but I like the early warning system

I am running a fully electronic ignition. Everything is set solid, and the car runs strong. It's just the tach. It's been bouncy since I had points in the car, but I will look it up.

......Checkthecoilwire?

I have had this happen to two cars. Once on my '72 914. When I changed the points and condenser, the tach steadied. (I had already changed the wires, cap and rotor.) Since you don't have points anymore...

The other time was a '76 Chevette, one of the early years of electronic ignition, (that lasted me 11 years and 188k miles with only tires, batteries, fliuds and 2 timing belts!) The local parts store suggested parking out in the country on a dark night and opening the hood. Wow! I had St Elmo's fire dancing along the plug wires and around the engine. With new plug wires the tach steadied nicely.

Your's may not be so dramatic but I'd suggest checking the wires if they aren't new.

Hope this helps somebody else if not you.

Waitaminit...I the above, I'm describing a jumpy tach on steady driving due to leaky wires and points. All my 914's bounce on downshifting but are steady on acceleration and driving. Figured that was normal.

That's what I have. If I can get it to act right on downshifting, I'm happy.

It's not bad, but it's not right. If I can fix it, I'll be a happy camper.

If you see the bouncing on downshifting or upshifting and not during steady operation, then the bouncing you see is actually oscillation of the needle when moving quickly from one RPM value to another.

This is quite common and expected with systems that are under-damped. The 914 tach is under-damped.

If you have a newer tach, you can add a capacitor across the coil of the tach to bring it to critically damped or over-damped and remove the oscillations.

I was successful in doing this with the tach from my 74 but not on the tach from my 71. The 71 has a much different circuit than the 74 and adding a capacitor changed the steady state RPM reading.

What year is your car?

It's a 72.

Any idea what size capacitor I need? I don't have a stock ignition or fuel injection to worry about.

For later tachs a 2200uF capacitor should work fine.

If you have an early tach and I don't know when they changed, you shouldn't add the cap as it will change the steady state value. For example, if running at 3k RPM, the tach might read 3500 RPM. This is how the tach on my 71 responded although it is nice not to have the bouncing tach.