bought this 1988 Johnson 20 hp with 1988 14' Lund. Third time on the water and each time it runs great at 100% power for a few minutes then goes down to 30%-40% power. Thought it was a fuel problem so purchased new OEM 6 gallon fuel tank and line/bulb. Running 91 octane with no alcohol. Then replaced the fuel pump with OEM equipment. Ran Marine Pro Sea Foam through it for 4-5 hours to get the gunk out of the carburetor. It was running great in my drive way at idle and high idle. Replaced the spark plugs with NGKs. The old ones did not look bad at all, but replaced them anyway. Got a spark 1/2" with both plugs. Today, took it out on the water and same thing...after 5-7 minutes the power went from 100% down to less than half that. I am truly stumped. Could this be a timing problem? Does the timing change after it warms up? Today, we could troll at low speed near idle for an hour without problems.
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