1975 merc 500 50hp runs rough and hesitates at over half throttle

[hardyct2010]

Hi all. Thanks in advance for taking the time to read this. I have a 1975 merc 500 50hp that I've finally got running. I cleaned the carbs and put new gaskets on them. The motor idles great but as soon as I add over half throttle the motor begins to run rough and hesitate badly. It acts like it wants to take off but it's bogging down. It still get the boat to plane somewhat though. Can anyone help me?

thanks

Chris