1975 Johnson misses on accelaration

[Venture4]

I have a 1975 Johnson 9.9 that was in storage for quite a while. I have rebuilt the carb, put a tune up kit (plugs, points, & condenser), replaced water pump, replaced the thermostat, and a new fuel pump was installed. The engine runs good at low speed but after running at slow for a while, it will stumble on acceleration and does not seem to develope full power. Also, after the new water pump was installed, when it is started plenty of water spits out of the relief port. Then a lot of bubbles seem to come out of the water discharge port on top of the cavitation plate. Could I have not installed the lower housing right and could exhaust be getting sucked back and causing the problems?

[Daviet]

Did you fine tune the low speed needle valve after you got the boat in the water?
Have you checked the compression, maybe you have a leaking head gasket.