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1997 Johnson 50hp motor bogs down after it gets hot

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Micahwhitlock92

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Hello, I have a 1997 Johnson 50hp outboard motor , we deleted the oil pump and have a straight vaccum fuel pump and batch mix. We have replaced stator, plugs, cleaned carbs , coils done about 5 years ago. The boat runs perfect for about 45 minutes on the water and then starts bogging down. You can have the throttle all the way up and it runs about 1/4 throttle. Unsure why the boat only runs like this after it gets hot after 45 minutes . Any help would be appreciated.
 
Do you have a tach to know what RPM this is occurring? This sounds like the engine is going into SLOW mode which limits the engine to 2500RPM when the temp sensor signals an overheat condition. When did you last replace the water pump impeller?
 
I do not have a tach, I’m unsure on the impeller it for sure hasn’t been replaced in the past 5 years. Would a bad water pump impeller cause the boat to run like this after 45 minutes or would it occur sooner and are there other symptoms the bad impeller would create that I could check for? Thanks
 
Micahwhitlock92 said:
I do not have a tach, I’m unsure on the impeller it for sure hasn’t been replaced in the past 5 years. Would a bad water pump impeller cause the boat to run like this after 45 minutes or would it occur sooner and are there other symptoms the bad impeller would create that I could check for? Thanks
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It all depends on what kind of running is done in that 45 minutes. A weak impeller can deliver just enough water in low demand conditions, but fail under higher RPM. Replacing the impeller every 5 years is just good insurance.

Watching your overboard indicator (pisser) is the next best thing to watch for correct water flow. As Racer said, checking your warning horn is a good thing as well. With the key in the run position, short your temp sensor to ground and you should hear your horn squalling. If not, your warning horn isn't working.
 
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