The manufactures try to find the correct balance of lubrication and carbon build-up in the motor from the unburnt lubricants, extra oil can cause carbon build up that can destroy the motor, it may also create more smoke and running issues. I'm not saying that's your problem, only that manufactures have decades of experience and have a much better understanding of what the oil ratio should be for recreational use than the typical boat owner. For high performance and racing applications sometimes higher ratios are recommended, but these motors tend to be torn down and cleaned far more often, so they can get away with it.
Break-in is a totally different application and is short term.
Back to the question.
Some motors have an RPM range where they don't run as smooth, frequently it's where the motor transitions from low speed circuits in the carb to mid-range or high speed circuits, not much you can do about it. With some work you can sometimes fine tune the jetting to help eliminate it, but you don't always get the desired results, and it takes a while to do.
The place to start is to check the compression, clean and adjust the carbs to ensure they?re working correctly, check the plugs, check for electrical/timing issues.
