RPM Instrument Jumping on cold startup

[fizeus]

Hi all,
I'm experiencing a strange problem I can't fix.
On cold startups (Car is in the garage for some days) I see RPM instrument jumping (it sometimes read 0 RPM!!) even if the engine RPM are steady.
Points has been replaced 1000 KM ago. They still seems new and dwell (measured when the engine is warm) is in spec.

The strange problem is that once engine become warm or after some minutes of driving the problem disappear.

Here is a video.

Please note that at 30 sec I gently pressed the gas pedal.

https://youtu.be/BGvur9xP_48

P.S: Car is a 1973 T with Zenith so it has no WUR

[Tyson Schmidt]

Recheck the point gap, and contact cleanliness.

Put a voltmeter on the battery to check charging voltage when you start it cold.

Check the connections at the distributor for the tack and points wire junction, as well as the coil trigger and ground connections.

I’ve also seen a failing coil cause this, but it’s usually when hot, not cold.

Finally, it may be the tach itself.

[JSV798]

My 1978 SC had a wildly fluctuating rpm gauge until I read on this forum that it could be the voltage regulator. Confirmed by fitting a digital voltmeter to the cigarette lighter which showed up to 18 volts when needle swinging. New voltage regulator fixed it.

[fizeus]

Thank you for the suggestions ... connections seem good also distributor points has been cleaned with lacquer thinner and some paper.

I was already looking at the VR .... On startups it seems to be a little out of specs and also while driving it continuously jump up to 15.5 Volts ...

Is this value too high or can I consider it in specs ?

[proporsche]

well? i would check your fuel mixture...1st..then your 15,5 is a little too much the max should really be 14,8....do you still have internal regulator or already external?
Ivan

[fizeus]

Ivan,
fuel mixture should be ok according to the Gunson ColorTune even if I think it's still a little bit rich.
My VR is an original external Marchal regulator.

I bought a new BERU GER 024 but I haven't still installed due to the modifications
needed to the fuse box panel (beer VR has different mount screws)...

to be honest I was looking for a used/damaged Marchal voltage regulator box to "hide in" the Beru VR box.

Suggestions?

Here the BERU VR photos

[jbell959]

I wouldn't be surprised if you find out that the board in the tach is causing this. I've rebuilt several of them and there are capacitors, transistors and diodes in these old tachs that go bad and can cause this.