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Stumped. 1988 Johnson 20 hp keeps dying after running great

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David.

New member
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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David. said:
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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Did you check the spark when it is failing?
 
As stated need to check spark when it fails..also need to clean carb if it had "gunk" in it as piece of debris could be floating around and blocking jet.
 
faztbullet said:
As stated need to check spark when it fails..also need to clean carb if it had "gunk" in it as piece of debris could be floating around and blocking jet.
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The spark has not failed. It has new spark plugs. The carburetor was cleaned with Sea Foam Marine Pro.
 
Confusing here.------Did you test for spark or assuming that with new sparkplugs it MUST have spark ??---Did you install a new water pump impeller after you bought it ?----Any other testing done ??-----Or answer this specific question.-----With engine cold in the morning does spark jump a gap of 7/16" on both leads , yes or no ?
 
racerone said:
Confusing here.------Did you test for spark or assuming that with new sparkplugs it MUST have spark ??---Did you install a new water pump impeller after you bought it ?----Any other testing done ??-----Or answer this specific question.-----With engine cold in the morning does spark jump a gap of 7/16" on both leads , yes or no ?
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Yes, it will jump 7/16” gap cold. Replaced spark plugs as a rule out measure; this was advised by mechanic. I actually checked spark with screwdriver. Did not install new water pump impeller. I am beginning to suspect a timing issue. Does the timing change as motor heats up?
 
Timing on that motor is set / fixed at the factory.-------It does NOT change for the life of the motor.----So no , it does not change when motor warms up !!
 
David. said:
The spark has not failed. It has new spark plugs. The carburetor was cleaned with Sea Foam Marine Pro.
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How do you know the spark is not failing if you do not check it at time of failure?Could have one cylinder dropping out?Does this motor have the high temp cut out switch on head?Whats the model#.
 
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