Usually if you have a problem with the tach. it points to rectifier issues. These usually don't effect the running of the motor, but all the timing of the engine is done by timing everything to the flywheel ad the rectifier is under the flywheel if I remember right. Something may have got burned or melted if you had a rectifier short out throwing off the timing. Just a thought. Is the motor charging going down the lake, or you just getting 12 V out of the battery. If it's not charging, I would check the rectifier ( voltage regulator) and see if it shorted out. This is common on small 2 stroke Mercs. My 50 HP merc. did it on ly 2 months old. Good luck with it! i can get you a # of a good mechanic in NC you can talk to about it if you can't come up with anything.