We have a 2000 model 496 CID 375HP Mercursier sterndrive in a 26 foot Formula boat. The 496 won't start. It seems to be starter flywheel related. It runs great when it starts. We put on a new marine starter and it worked for a while and then when it would not start we could turn over the engine a little with a wrench and then it would start. Then later it would not start at all. Put on another new marine starter and nothing at all. It just sounds like the starter is grinding against the flywheel. We were able to turn the engine over 360 degrees and there does not seem to be any flywheel teeth missing. Any thoughts? Thanks a lot.