Now that I've done extensive work to get the V12 running smoothly, and fixed the AAV so I could adjust the idle, I noticed that the tachometer must be not reading accurately.

If I adjust the warm idle to 750 rpm as shown on the tach, it's just too high. I can tell by both the sound, and that it's too anxious to go if I let off the brake. I would venture a guess that it is off by about 250 rpm or so.

Is there a way to calibrate the tach? Or is there something interfering with a proper signal?

Thanks.

 

Mac
The first thing is to know accurately what revs the engine is doing. You can buy a gadget that clips to the HT leads, or somewhere anyway, that besides beings being a timing light reads off the revs. Garages have them too, so some friendly place might help. Then you will know what the problem is, if any. FYI, on my car about 600 rpm feels right for the tickover.
Greg

 

I agree with Greg.

Mine read about 20% low, and that is just a guess.

The massive rewire of that car made no difference.

I spoke to an instrument repairer I know, and they were happy to recal the unit, but I never really got the warm and fuzzies strong enough to do it.

My ear is good enough for me.

When I got the "boot in", I am seriously watching where I am going more than what the gauges are doing. The speedo is the only one that catches theeye.

 

Thanks Gentlemen,

In thinking about it, the only reason I care about the accuracy of the tach is to properly set the idle.

With the TH400, it's not like I ever have to worry about redline in normal driving... or anywhere near it. So if the tachometer is off by a small amount, I can live with that.

So, all I need to do is to determine actual RPM at idle regardless of the tach reading and adjust accordingly. There are lots of gadgets available to that. A very cheap alternative is a laser non contact tachometer:

Digital Laser Photo Tachometer Non Contact RPM Tach Meter Motor Speed Gauge USA | eBay

Alternatively, I just found that there is a free app for the iPhone that turns the camera flash into a strobe light that you can tune to the proper frequency of the desired RPM. For instance 11.67 Hz is 700 RPM. It isn't a true tachometer, because you'd have to fiddle with various speeds until your mark looks like it stopped moving, but it will accomplish the task with some trial and error.

I think I'd have to do it from under the car to get a clear view of the crank pulley. From above there are so many moving bits it might be hard to tune it to the right frequency.

 

Thought I'd update this to report that there is a way to calibrate the tach.

I noticed on mine that after shut off, the needle would fall well below zero, and incorrectly assumed it jumped to zero on start up. However, after watching it carefully on start up, it looked more likely that zero was where the needle parks after shutoff.

I decided to take the instrument cluster out and take a closer look. After taking the tachometer completely out of the cluster, and turning it over, you'll find a small plastic lever that allows you to adjust the needle. It's a delicate adjustment so be careful.

I don't know if this is true of all model years since I can only confirm it on my 1990. However, my frustration of trying to figure out my true idle speed is relieved.