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25hp Johnson running on 1 cylinder?


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We have an 82' or 85' (can't remember) 25hp Johnson. On Saturday we went duck hunting and it was only running on 1 cylinder. We bought new plugs and Sunday it ran fine until we were coming back to the landing and it was down to 1 cylinder again. It doesnt look like the new plugs are fouled. Could this be the coils or is there an easier, less expensive place to start?

Thanks for any help!

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Well my dad (retired) ran around today to talk to some mechanics. We found out the motor was a 94 from serial numbers. We had been having trouble w it spitting water and the guy said in the early 90s Johnson started putting a computer in that would shut the cylinders down if it started overheating. That shop didnt have the pump we needed so he went to another shop that had one. We replaced the water pump tonight.

He did replace the gas and remixed the oil w no success.

My dad also found out he was testing the coils wrong so when we take the boat out tom we will be testing that also.

If anyone else has suggestions feel free to chime in!


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Took the boat out today. Pumps water like it should. Doesn't just drip drip drip. It ran fine for a while and then started doing it again. It sounds like it might have to do with that SLOW system.

She runs good when going about 1-4 to 1/2 speed but won't exceed that. The motor does start shaking like it miss firing but once let off the gas it's just fine. From what I can tell that SLOW system has many components. Any suggestions would be much appreciated! We might just have a 10hp from now on instead of a 25 eek

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Not sure if related to your issue or not, but i had a '96 25 and found when I rebuilt the carb that the fuel pump had a crack in the diaphragm. I rebuilt that too, and after installing the float right side up (doh!!!!) it ran great afterward.

Pretty easy to rebuild both carb and fuel pump on this motor, just remember to put it back together properly, haha!

On mine, remote steering, it would sound an alarm when it was overheating. Water was still coming out of pee hole, but not getting enough at trolling speed was my original problem. Good luck.

edit - I know the fuel pump is no where near the carb, but was looking at both as possible culprits. smile

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Thanks for the suggestion. We were thinking about taking the carb off.

With our fishing boat kicker we rebuilt the carb on a 5hp mercury earlier this year. The 2 situations don't seem the same. the Johnson runs perfect at low speed and runs good before it heats up. The mercury ran horrible at low speeds and eventually chit out. We were thinking about taking the carb apart and look at It. I'm sure it could use a cleaning anyways grin

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