Is your motor's charging system regulated?
There is a good chance that it isn't. Especially if you have an older motor. Systems without a regulator get regulated by the battery. A dirty or loose connection at the battery or a bad cell in a battery can cause voltages up to 18-19 volts. Our electronics do not like that. If you are lucky you have gear that will warn you of an over-voltage but not all have that feature. I learned this the hard way by going thru 3 Garmin 250s a few years back. I kept sending them back and Garmin Kept sending me replacements. The problem was only found after I had installed a volt meter on the dash. They are inexpensive and very easy to install.
While on the subject, if you have ever hooked up your battery cables back wards, even for a split second, there is a 99.9% chance you have blown your rectifier and your system is not charging.
Just thought I would pass that along and hopefully prevent someone from damaging there electronics from it. I've worked on a lot of motors but never a 4 cycle. So I don't know if they are regulated or not. Seems like they should be but I can find out.
Fair Winds and Following Seas
Bill H. PTC USN Ret
Chesapeake, Va