I was running my boat yesturday good for one hour then it started making a clonking sound then stalled, I was only doing around 2000rpm. now it will not start. it turned over a few times ok but no start. After a few attempts the fly wheel would turn but the engine stopped turning. I notticed the key where the fly wheel sits on was broken. The engine still turns so its not locked up or anything. but what was the clonking sound? as soon as the sound came it stopped working.