Porsche 917-3, Goodwood. The gauge can only jump among very large steps. What might be happening here?

I have no idea, but for only Pound Symbol 1600, (Real money $4000) you could find out...
http://aberdeen.craigslist.co.uk/ptd/5054452516.html

In 2013 video it does the same:
https://www.youtube.com/watch?v=zmbJzXrgasU
To me it looks not like moving in "discrete positions" but more like that it updates only every second...strange.

Mechanical tachometer being worn or rusted or lack of lubrication. For example:
http://www.motorcycleforum.com/showthread.php?t=94629

Quote from Hyperactive :

Mechanical tachometer being worn or rusted or lack of lubrication. For example:
http://www.motorcycleforum.com/showthread.php?t=94629

Are you actually sure about this? This tachometer behavior seems really common with old Porsche race cars, mainly the prototype and Le Mans competition vehicles. I've seen it so often, and always in Porsches that it makes me wonder if it's actually designed like that. I'm pretty sure I've seen some historical period videos with them doing this back in the day also.

Funnily enough I have tried to google about this multiple times, but no info on the matter seems available.

-EDIT- Apparently I have finally made some progress with the googling. These are called chronometric tachometers and they are supposed to move like that. Thought why they are so common with old Porsches, that's another story.

How can something be designed to do that? I don't understand why this isn't some kind of (design) flaw.

It is definately by design and not a faulty gauge, it is in too many videos.
"chronometric tachometers" was good cue!
Here at end is some description http://www.howardinstruments.com.au/page/HRD-Vincent/

It does not really explain its advantages or why it was still used so long but this is interessting:

Quote :

The movement of a chronometric speedometer is strictly an integrating mechanism, the controlling factor being the time base, a conventional ‑ or near conventional-escapement unit as used in every watch and clock mechanism. In this instrument there is no free movement of the pointer since at any instant it is, either rigidly locked in a given position or geared direct to the driving, cable.

In "normal" tachos the needle is moved by magnetic field and a spring, which is actually quite "freely".
So the chronometric gauge might be less disturbed by vibrations and easier to read?
Or maybe for some time it was just more accurate than other systems at its time.

I can not imagine that when the 917 was built it still was really huge advantage so perhaps tradition also played a role to keep it so long?

Oh wow I would have never thought it was supposed to do that on purpose .

Quote from Gutholz :

So the chronometric gauge might be less disturbed by vibrations and easier to read?
Or maybe for some time it was just more accurate than other systems at its time.

I can not imagine that when the 917 was built it still was really huge advantage so perhaps tradition also played a role to keep it so long?

Yeah just after posting yesterday I read a little more about this, it was described that these types of tachos are quite stable under "hars conditions".
If you have ever seen how some old skool magnetic tachos react (slow, lagging behind, reading getting disturbed by g-forces etc.) it starts to make sense. Actually in my own car the original tacho reading fluctuates with cornering forces and it's not even that old ('86).

Chronometric gauges have become obsolete for obvious reasons, but even modern tachos tend to suffer from erratic behavior from time to time (at least the ones you don't have to pay top dollar for) so these things start to make sense to me. I still prefer the modern way tho.