76 25 hp evinrude running on one cylinder

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.

Re: 76 25 hp evinrude running on one cylinder

You might have a bad set of reeds or an air leak into the crankcase, Thai. <br /><br />Inspect the reed plate, the carb and manifold gaskets and the bypass plate gasket<br /><br />Have you checked compression? Very low compression could also cause a cylinder with good spark and fuel to fail to fire.<br /><br />Good luck.