I have a 1982 35hp that idles perfect when its first fired up, even revs up and drops to idle without incident, but after approximately 10 minutes of running, it doesn't want to idle down without just shutting off. The shut off point seems to be just as the cam follower passes the timing marks. Sounds like its just loosing all spark. It fires back up after this occurs, but sounds a bit rough at low speeds, an refuses to idle down without shuttin off. The carb is spotless and adjusted correctly. I've tuned it in about 3 times, then took it out only to have it do this each time after it was ran for a short period. Is this caused from a bad coil, or something else I may be overlooking (incorrect timing?)? Damn thing keeps bluffing me into thinking its fixed. Any ideas?