Hello, and welcome to Justanswer. My name is XXXXX XXXXX I will be helping you today. If you need clarification on something please post back here.
Mercrusier uses quite a few ignition systems in these engines. I would need your engine serial number to see which ignition system you have.
Then what you have is either a bad;
1. pickup in the distributor
2. ignition amplifier
3. ignition coil
To test it properly and not buy parts you don't need, you have to follow the troubleshooting flowchart. I have it here for you.
Attachments are only available to registered users.
Just follow the chart step by step, without jumping around, and it will lead you to the problem.
Post back with questions.