A friend of mine has a 01 290 sundancer it has twin merc 5.0 EFI's in it. He has one tach that is off 600-800 RPM's off from the other tach going off the sync gauge. The sync gauge seems to be right according to the way the motors sound. Where can we start to check for the problem? He has a potential buyer and the guy wants that fixed. Is there a way to recalibrate it or something. Thanks for the info.

Well I'd say swap the signal wires and see if it throws the other tach off. If not replace the tach or see if is has a 4, 6 or 8 cylinder setting on the back. I doubt it but if it did and was set wrong; it would cause it to be off