So we inherited a 2008 Boston Whaler - 13' and it is a dinghy of sorts for us it has a 40 hp - 4 - stroke EFI on it - always been in freshwater. ... I decided to post in the motor section... When I got it changed the fuel filter and oil and have run Mercury Quickleen through it. Motor always starts great. Runs fine on plane. It bogs down at 2500 to 3500 rpm's - the harder you accelerate the more it bogs. The slower you accelerate the better it gets on plane. Once on plane it is fine. I don't know the history. Spark plugs are next. What else should I be looking for?