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Pre 2018, the max hp for the Montauk 170 was 90hp.

A prior owner repowered after the fact and was too lazy or stupid to upgrade the harnesses and gauges.

I would have the rigging inspected for compatibility. If they are re-using the old it may not be displaying correct info.

I find it highly doubtful engine is going 6k at 26 mph.

Go to a Merc dealer (Nauset Marine? )or an independent shop that has Mercury factory trained mech's.

The alarm would come from the key switch on the old rigging.