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Re: Tach question - experts?
Chuckles, with a tach reading off the flywheel sensor, you shouldn't have any interference with engine speed.
The wires on the sensor are just counting pulses, plugging and unplugging that should make no difference in how things run but do try that to verify. Then there should be two power wires for the tach, and that should just be seen as another 12-volt device, nothing special, no way to change engine speed by plugging and unplugging them.
It sounds more like you've got some kind of accidental interaction, that the tach is pulling voltage or creating a voltage loop somehow. If you had exotic electronics it could be a magnetic interference issue but I don't think Yanmar has blessed anyone with that stuff yet.
If you fire it up with the new system, and then disconnect the sensor lead to the new tach, or the power lead to the new tach, does connecting/disconnecting either of those allow your engine rpm to go up all the way again?
Is there any chance that some of the new wiring is simply blocking your throttle cable, or something "it can't be that simple" simple?