Also check to see if there is a sector switch on the back of the tach under that mounting plate to tell the tach how many cylinders you have. Our tach was doing just what you described, turning the selector switch back and forth a few times between 2 to 8 cylinders cleaned up the contacts and it immediately settled down when I switched it back to 4 again. Has been fine since. (4 years now)
Funnily enough, there was a comment in the survey
that the engine
idle speed was way too high at 1200 rpm
. Of course, this was caused by the same switch playing up. Not so sure about the survey
guy, it was pretty clear the tach was wrong to my ear.
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