I tried the "inertia spin" but couldn't determine anything definitive. I opened up the bezel and glass and was able to swing the needle to red line with no mechanical obstruction. So I put it back in the car and now, mysteriously, it will climb with the rev's beyond 1500. I momentarily rev'd it to up to around maybe 4000-ish and the needle followed however it did quirk out jumping up and down in the 3000-to-4000 area. I have two extra tachs and all three of them quirk out in kinda the same way. I'm starting to wonder if the ignition pulses are not clean. I'm still running points but I had to replace the coil - it currently has one of those blue Hecho en Mexico Bosch coils. I think I have another original black coil in the parts heap that I could try, but it's 45 years old. I just thought I could put the timing light on it and see if I can detect any skips in the spark.

Moving on, i had a situation where the engine temp gauge was pegged out max with a cold engine. The sensor's resistance was only about 1 ohm so I swapped in a different sensor. This one reads about 4500 ohms when cold and now the temp gauge doesn't seem to move off the cold line even after running the engine a few minutes. Does anyone happen to have a temp vs resistance curve or at least a couple data points that I can use to check it?

And the saga continues....

Thanks again!