Tach needle starts wild jumping above about 4,000 rpm

[AlaskaFestivaGuy]

I just replaced coil, wires, and plugs, fixing a long-standing hesitation/stumbling problem.

Now, whenever RPMs get above about 4,000 rpm (climbing mountains), the tack starts wildly jumping, dropping instantly down to around 1,500, then back up. As soon as I back off RPMs by a few hundred, it settles back down, but then doesn't drop below about 1,200 even when back to idle.

I don't recall this happening ever before the above work. The engine obviously is not doing what the tach needle indicates.

The tach must be electrically driven (not from speedo cable). Loose connector (I didn't disconnect/reconnect anything)? One of the wires coming out of the distributor?

I also had the engine compartment steam cleaned last week -- perhaps water is still stuck somewhere?

Perhaps whatever electrical component generates the tach signal to the needle (on airplanes, we call it the "tach generator") is starting to fail?

[zoe60]

The tach signal comes from one side of the coil. I can tell you which side after I get a chance to look at mine. That wire may be loose or have another problem. Since you just replaced the coil, I would suspect that.

[denguy]

Need good grounds.

[AlaskaFestivaGuy]

Right … I did disconnect the wires from the coil when I replaced it. I made an effort to clean the contacts, particularly the "U"-shaped part that attaches to the coil's positive terminal before transferring it from the old coil to the new coil.

[Rick the Quick]

Maybe high humidity caused the water from the steam clean to linger. I once used a pressure washer on an Audi 5000 and it took a week before it would start. Perhaps a shot of WD40 would help displace any moisture.

[WmWatt]

The tachometer counts the number of spark impulses from the coil and divides by four.
I'd disconnect the wires from the small terminals of the coil and wipe down ends and wires with the usually recommened clean dry rag.
Yeah, the tach wire is not a spinning cable like the speedo. No kinks or drying out of lubricants.