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Seems like the timing is off. Maybe not advancing the timing.
Also sounds like its not getting fuel. I would check the carb. I am an auto tech, but boat engines are my hobby.
If you are getting spark its not not the power pack. You could try to test it on the 30hp. If the power pack was bad you probably would have no spark. Have you checked the keyway on the flywheel?
Have you gone through the carb? I have seen jets and other things looses up in the carb causing a similar problem to yours. Also do a compression test on all cylinders.