I have always had to use two batteries for starting my Yanmar 3GMF, and something happened three weeks ago that got me thinking. A power boat friend got towed back to the marina because of a fried starter. Long story shortened, his problem was a bad GROUND cable, the crimped ends just pulled right off! That possibly caused his problem with the starter, and after replacing both he has no more problem. So I thought, I never checked my ground, I checked, and found that I too had some corrosion between the cable and the engine block, cleaned that up with the Dremel(r) and now have the starter that I never had before!!! I have always cleaned the battery terminals, but gave no thought at all to the other end! Starter spins as if the compression is released, starts so fast you barely need to push the button. Check yours, the cable fastens behind the engine near the starter housing. Good luck!