'bouncing tacho?'

[twin plugged targa]

Car: 1970 911T standard.

Indicated revs seem to bounce occaisionally when at low revs in gear around 2K revs or lower.

Once on steady revs the needle is accurate and steady.
Not sure if this could be a loose connection or a sensor?
Maybe someone could point me in the right direction?
Many thanks for any help
regards Ben

[cashflyer]

Bouncing taco?


No problem.
You may need to sneak up on it, but once the taco is steady, grab it with both hands.
Then, enjoy the tasty taco.



***

Does the engine run smoothly at an idle, or is it rough?

[Hugh R]

Don't know your car, does it have a cable drive to the tach? If so, then its usually in need of graphite lubricant inside. Old cables grab and whip causing the bounce and steady out under acceleration.

[Gogar]

If you still have points, make sure they are up to snuff. Check your alternator/VR voltage to make sure it's not too low or too high. Check all the important ground spots on the engine.

[twin plugged targa]

I will look into these replies, except cashflyers as I don't have any salsa at hand
I hope it is not the points as the car is fitted with the dreaded original Marelli unobtainable Distributer, points, rotor and cap.
I was hoping the distributer and components might last a bit longer as I am hoping to sell the car shortly and they are in immaculate condition (appearance wise)
The engine is believed to have covered 32K miles and all components appear to reflect this mileage. However I do understand that what may appear to be immaculate may still be faulty.

I would not think it could be grounding points as the car has just had a bare metal resto a year ago and is dry stored/used 1k miles since resto.

Voltage regulator could be an issue I guess with alternator at lower revs.
Could I just read voltage at battery live (at lower revs to check this? or any other reccomended areas?

What voltage range should I be getting?

Is my tacho cable drive? if so, where can I lubricate it?

Thanks
Ben

[Paulporsche]

I had this happen years ago. Turned out the points were ready to be replaced.

[wwest]

If you have the CDI, Capacitor Discharge Ignition, then over-voltage at the alternator output or, OR..over-charging of the battery might be the cause. When it starts bouncing turn on as much electrical load, headlights, etc, as you can and see if it settles down.

High voltage at the alternator output might mean poor electrical connections all the way up front to/at the battery. Over-charging the battery itself would indicate that the connections are good but the VR or the alternator is wacko.

[Walt Fricke]

Your speedo is mechanical drive. But your tach is not, so nothing to lube.

The best way to check voltage is to purchase a voltmeter which plugs into the cigarette lighter. Checker Auto has them, so other FLAPS should too. You can use on any car, so it doesn't have to go with yours when sold. Always a good thing to have so you can catch poblems while they are starting.

Tachs have some kind of mechanical damper on the needle movement which can be tightened. But I don't know how. Kind of thing usually done by an instrument shop like North Hollywood or others who know what they are doing. Not that this is necessarily your issue.

Points can be filed and readjusted so they work just like new. For a little while, anyway.

[DanS911]

While you are at it check all ground connections that you can find......amazing what can go whacky from a bad ground.

[piggdekk]

I have a similar problem with my 71 T, in fact I couldn't figure out why my idle was bouncing when the engine was hot. So a technician disconnected the tacho and the issue was gone, my idle now is very stable. When the tacho bounces, does the idle bounce as well? Now I need to figure out whether the problem is caused by the tacho itself or by the wiring from the coil. In our model the signal doens't go straight from the coil to the tacho, but first it goes into a small box where the voltage is reduced. So the issue could be the wiring, the tacho itself, or this small box.

[twin plugged targa]

Dear all,
I do not have CDI ignition (I believe that is non stock?)

My engine and ignition is 10% stock (I believe).

The Revs are stable and do NOT fluctuate - only the indicated revs on the tach. are fluctuating.
The car runs fine, idle, acceleration, etc are all fine.

I will measure voltage across the battery and confirm stable voltage (or not)

Many thanks
Ben