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If it is an 83, you need to look into the points set an condensor. The ignition system works on an old fashion sysem, no different than 1955 automobiles. Points and condensor should be changed yearly regardless of hours, as the ignition system actually "wears and tears" and eventually will quite working with age and engine hours.
If you go by the book, Items such as points, condensor, cap, rotor and plugs should be changed yearly, again, regardless of hours. Spark plug wires every 3 to 5 years depending on useage and climate.
There are a few other things, but this is the most likely problem, an old fashion tune up. Start with this and if it does not fix it let me know and we can dig further.
Post right back here with any additional questions or concerns my friend.
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Mercruisers have Thunderbolt Ignition which do not have points, rather electonically fires from the optical, I think, signal from the distributor timer attached to the rotor.
Perhaps some things in the Thunderbolt system need checked as well as the distributor,etc for dirt and cracks?
I could be wrong, But it did not think thunderbolt 4 ignitions came out until 1985ish. Before that was points.
No problems though.
Here is a flowchart to troubleshoot that ignition system.
Follow the flowchart and it will help you narrow down what it is. The distributor uses a hall effect sensor inside of it. On the older ones the steel core was exposed, and they rusted up pretty good. (newer ones and replaces sensor are all encased in plastic so that does not happen) When you go to look at yours, if your sensor is a big ball of rust you can almost be assured that either is, or part of the overall problem.
Post back here with any additional questions or concerns my friend.