Re: 1979 Johnson 25hp overheating
The spitting water tells you that the water pump is functional, but that's it. The flow of water through the cooling passages of the engine is controlled by the thermostat and if it is blocked you cannot tell it except by the temperature of the block. The water being discharged will vary in temperature as the thermostat opens and closes if it is working at all.
You could examine the thermostat housing and test the thermostat in a pot of hot water. It should open and close at the correct temperature range. Use a kitchen meat thermometer - it covers the 143 degree range.
The thermostat is located at the top of the cylinder head cover so that you can access it without removing the cylinder head. You should replace the gasket.
You can see the diagram and get the part numbers at the BRP parts catalog site, "cylinder and crankcase" diagram:
http://epc.brp.com/default.aspx?brands=ej&lang=e
