Depends on how old it is. I had a boat fitted with 2x75 mariners that had not been used for about a year prior to purchase. The port engine did much the same thing as yours and when stripped it was found that the main lower bearing had started to breakdown. Fixed that then the s/brd one chuched it
. Certain Mariners have a problem with the flywheel magnets coming loose and damaging the stator plate, if you run your finger around the bottom of the flywheel you can usually find bits if magnet.