I have a 76 25hp evinrude in pretty good condition that has shown a loss of power. Use about one hour and have to replace the spark plugs. I have cleaned the carbs, checked the spark plugs and both spark strongly, replaced a leaking head gasket, and still only running on one cylinder (the top one). So, I switched the spark plug leads(? sorry if i don't have the right technical language, english is my second language) and the top cylinder still didn't run but the bottom one did. Even after switching spark plugs and the wires (from the plugs) too. the bottom cylinder always runs, even when using the spark plug wire from the top cylinder. any idea what it could be? thanks and let me know if you need any further info.