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Hello Robert. My name is Tan.
This will require testing when the engine is acting up. You will need a timing light and someone to run the boat. There are a couple of things we can do before the timing light test to eliminate a fuel related issue. Run the boat till it acts up then push the choke in. Does the engine pick up and run better? If no change when engine is acting up squeeze the primer bulb real hard. Does engine run better?
If no change on both of these tests we need to do a spark test. Connect the timing light to the top spark plug wire run the boat till it acts up. Watch the flash of the timing light. You want to see a steady flash with no dead spots. If no misfire move to the next cylinder. Do this test on all cylinders. If you find a misfire swap that ignition coil with a known good firing coil. Does the misfire move? If the misfire moves then the ignition coil is bad. If it stays we have another ignition component failing. This will require testing with a peak reading volt meter to find the failing component.
Can you do these tests?