I have a 1976 mercury 500 serial # 4428230. This boat sat for about 10 years when I bought it 2 weeks ago. Before I took it out I changed the following ,lower unit oil, water pump impeller, fresh gas and good oil from the marina, new fuel line and primer bulb, changed plugs. The problem I am having is an intermittent power loss at wot. 
Sometimes the motor will run great at wot and plane the boat. But most of the time you can run a 1/2 throttle fine but when at 1/2 plus it acts like it is bogging down. I can choke it and it will pick up a little bit. It runs smooth but just no power. I replaced the switchbox because I had a spark loss on some cylinders and it fixed that. I have also put new gaskets and fuel pump diaphrams in the carbs. Replaced all fuel line in the system. The primer bulb does stay hard when it is acting up. And compression is 125-135 on all 4 cylinders. Any body have any suggestions I have a manual and am fairly mechanicaly enclined. Sorry for the long post but wanted to try to cover all the bases.
Sometimes the motor will run great at wot and plane the boat. But most of the time you can run a 1/2 throttle fine but when at 1/2 plus it acts like it is bogging down. I can choke it and it will pick up a little bit. It runs smooth but just no power. I replaced the switchbox because I had a spark loss on some cylinders and it fixed that. I have also put new gaskets and fuel pump diaphrams in the carbs. Replaced all fuel line in the system. The primer bulb does stay hard when it is acting up. And compression is 125-135 on all 4 cylinders. Any body have any suggestions I have a manual and am fairly mechanicaly enclined. Sorry for the long post but wanted to try to cover all the bases.