Why would a C150 tachometer be sluggish? After having an issue with a tachometer bouncing so much that it fell off, I now have the opposite problem with the replacement (Mitchell D1-112-5025): The tachometer is sluggish. When I open the throttle from 1,000 to 1,700 for the runup, the tachometer takes about 6s to show 1,700 RPM. The delay seems even more pronounced when I reduce power. The smaller the change in RPM, the longer it takes the tachometer to update.

My A&amp;P says that it just needs breaking in, which could possible because the sluggishness has reduced since the tachometer was installed; however, I was under the impression that modern tachometers should work without sluggishness immediately.

The counter increments as expected.

How should I approach this problem? Is it common for tachometers to need >15 hrs to break in? Is it possible for a drive cable to cause this issue (continue to spin, but in such a way that the tach speed is sluggish)? Could some kind of installation issue have caused this problem?

UPDATE Mar-22: Removed the tach cable, inspected outer and inner cable, washed, dried, lubed with graphite, and replaced. Cable seemed good, but the tachometer is still sluggish.

UPDATE Mar-24: Tachometer is now beginning to exhibit the same symptoms as the previous tachometer before the needle spun off; the tach is still sluggish, but the needle is now bouncing considerably.
No you don't have to "break in" flight instruments.

When a TSO'ed instrument is new it will get an Acceptance Test Procedure (a functional test) from the factory done before it is released and it has to perform to specification.  There is no such thing as a break in in service.

That being said, sometimes parts get by the ATP and malfunction in service, so you never know.  In my years in the regional airline industry, "bad from stock" is not uncommon even with super fancy Transport Category components.

Problems with the flex drive could be a factor, but a flex shaft would have to be binding and winding up a hell of a lot to create that kind of lag.  It's still something that could be checked.

Before that though, you should call Mitchell's tech support and describe what it's doing and see what they say.   Maybe they'll tell you there is a break in, but I'd be pretty surprised if they did.