When you say spins do you mean the engine turns over but does not fire, or that you can hear the starter motor spinning but the engine does not turn over? The former condition has a pretty complicated logic tree, the latter condition is pretty simple; a bad starter motor. A bad starter motor is good and bad news. The starter was less than $200.00 when I bought a replacement five years ago. Why I bought the starter is kind of the bad news. At the time the engine was out of the boat having a new transom assembly installed. The service manager told me if the existing starter ever went in the future, I would be on my own as he had no intention of ever pulling my 496 out of our boat again. Basically the 496 is a monster. It has to off the mounts and lifted a couple feet at a minimum to get access to the starter. But because of the space (or lack of it) working on it in the boat is grossly unsafe. So it has to come all the way out.