Re: Just purchased 89 19' capri?
You have a problem the boat should be turning around 5-5500rpms. I doubt it is overproped because being just purchased prob has a prop that matches the boat(unless the owner kept the right one)I would start by rebuilding the carbs and running a compression check. if the problem remains follow the lines back to the tank and check for obstructions and replace fuel lines and spark plugs. check each cyl for spark to make sure the power pack is functioning(distributor for boats).. then check the fuel tank for foreign materials,water,and drain all the old gas out. try to do everything you can yourself you will need this knowledge down the road with an older boat.
once the engine is healthy i could see that boat running 30-40 mph if propped correctly just remember that speedos on the water are not accurate(verify with gps).