Bad tach, or ?

[Mastiff]

I just recently noticed that my tach (cheapo sunpro) occasionally flicks up about 100 rpm and then back. I can't feel any roughness. The weird thing is that it'll do it at idle, but also at 1500+ RPM and the behavior is the same. I've never noticed it before, and the way it does it at higher RPM doesn't seem like it could possibly be related to the actual engine RPM. I'm going to trace the wires and make sure nothing is coming loose, but do tachs ever just wear out? This thing is about 20 years old.

 

[nvrenuf]

In my experience, they work or they don't. I vote connection issue.

 

[Mastiff]

Nevermind. I went out to just take a quick look if something was obvious. Somehow a plug wire fell completely off and was laying on the header. Not melted, or shorted, just off. I can't explain how it showed those particular symptoms, but the twitching is gone.