1999 70 hp johnson low rpm no power

bneely

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Jan 29, 2012
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I have a 70 hp johnson outboard, model number J70PLEE. It starts well, idles well, and normally hits top speed at about 37 mph turning around 5000 rpm. Yesterday it was cold...around 22 degrees F. The motor fired up, I let it warm up for about 5-6 minutes, and we headed up the reservoir like normal. I took it off plane and idled it around for about 10 minutes looking to mark some structure and then tried to put it under load again and all I could get was about 20 mph and 3000 rpm with the throttle wide open. The motor sounded a little different, but wasn't running rough. Did I lose a cylinder? and more importantly, how do I get it back?

I pulled the spark plugs when I got home and none were fouled or worn...I did notice that the plug out of the bottom cylinder appeared dry compared to the top two cylinders. I plan on changing the inline fuel filter, and picking up a compression tester to double check the compression in each cylinder, but I have suspicions that a carb clean-up is in order. Any other thoughts?
 

sutor623

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Re: 1999 70 hp johnson low rpm no power

Awww man. Do you think that the bottom cylinder ran dry due to a carb blockage? How does the motor run at lower speeds? How many carbs are on this motor, is it one or two? Get a compression tester as you said and rule that out first. Also, check your linkages and see if the throttle linkage somehow loosened up. I hope for you that it is not a fried cylinder. :eek:
 

sutor623

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Re: 1999 70 hp johnson low rpm no power

Also, do you run seafoam in your fuel?
 

bneely

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Re: 1999 70 hp johnson low rpm no power

The motor runs fine at low speeds, and starts and idles fairly well. The compression test looked good... 125, 120, and 120 on the 3 cylinders, top to bottom. I do not run seafoam in the fuel, but I do add the OMC/Johnson 2+4 fuel conditioner, and I run the boat all year, never letting it sit more than a couple weeks in the garage between adding new fuel and additive and running the motor. I replaced all the spark plugs and I'm going to add some seafoam or kleen(suggested by another friend) into the fuel and then I'll take it to the river to test it out this saturday. I may change out the inline fuel filter as well.

As for the carbs, there are three, and I am hoping that it is something simpler to fix than a blockage, but it is definately a possibility. I will try the simple things first and if new plugs, carb cleaner, and a warmer day to operate don't help, it may be time for a carb disassembly and inspection. The thought has crossed my mind that the choke froze shut seeing as the temps were so low and the motor hadn't been running long when we dropped to an idle. It might run better just taking it out at 50 degrees vs 20. Any thoughts on this?
 

sutor623

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Re: 1999 70 hp johnson low rpm no power

Dude, I have had some really bad experiences being out in the cold, so yes running it on a warmer day may be the ticket. Do your carbs leak fuel at all? (Especially for me, my motor does NOT have a thermostat, so getting/keeping my old '67 Merc warmed up can be a chore on a 20degree day) Congrats on the good compression too!! If she starts and idles well, we can rule out water in the gas line. Seafoam often cleans out my carbs. I run about 1/2 small bottle in a gallon of fuel/oil mix and let her idle, and then give it some gas, idle , give it some gas, then let the motor sit for 15 minutes, allowing the seafoam to de-varnish the carbs. Then run the rest of the fuel (1 gallon) out of the tank. Make sure to run a regular mix of gas/oil into the motor though, you dont want the heavy seafoam mix to sit in those carbs. And be careful, if one of your carb jets has some debris that got lodged into it (often from old, breaking down fuel hose thanks to ethanol in our gas) and there is a cylinder or two running lean, you will be asking for trouble. Funny thing is, that if the choke was froze shut, calling for more gas (pushing the throttle) isnt going to prevent the motor to rev higher, typically the motor will bog out due to flooding the cylinder with too much gas/oil and not enough air. So I doubt it was the choke. Check your throttle cable with the cowl off and motor off. Push the throttle all the way down and see if the carbs are fully opening, and if the ignition system is advancing properly. If not, you have a throttle linkage issue, and that is why the motor wont go full throttle.

If all these tests prove to pass and not change anything, we will start looking into the electrical/ignition system. From what you are telling me about the motor running fine and just not going higher up in RPMs, I am betting that your throttle cable has either slipped loose, or is going bad. Let me know what you find. Peace.
 

bneely

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Re: 1999 70 hp johnson low rpm no power

I appreciate the feedback on this site. It has been very helpful. That being said...I'm still having frustrations with my 70 hp johnson motor. Over the past 2 weeks I took it out and ran it 2 different times with fresh gas and a can of seafoam in the tank. Both times it started and idled fine right off the trailer. Both times it jumped up on plane at full power and down the lake we ran. Once I ran about a mile before stopping to fish, and yesterday I ran about 15 miles at full power before shutting her down to do some fishing. In the first instance, when we fired the motor back up, it was the same old story...only 3200 RPM and 20mph max. The motor doesn't smoke or knock, just won't turn the RPM's it should at WOT. Yesterday, it ran great on the ride up the lake. We stopped to fish for 20 minutes or so, then re-cranked the motor and took off. HURRAY, we thought...finally full power after a stoppage and restart. Our celebration was short lived however, as about a half mile into our next run, the motor abruptly dropped to 3200 RPM and 20 mph, nearly unseating us. Again, no abnormal noises or smoke.

The intermittent nature of this problem has got me thinking electrical problem. A friend of mine suggested it could be the powerpack or the associated S.L.O.W. system kicking in. I suppose a bad temperature sensor or something could trigger the S.L.O.W. system, but I haven't gotten any alarms or lights on the dash indicating a problem. How much does it cost to replace a powerpack, and is it something I could tackle without taking it to the shop? Thanks for any help in advance!
 
Joined
Jan 17, 2011
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Re: 1999 70 hp johnson low rpm no power

Are you sure your waning horn works?.....if your in slow you should get a warning horn. You can test it by unplugging the tan bullet connector at your temp sensor, then ground the tan lead---wireing harness side to the motor ground. You may have to use a small nail or pin because it is usually a female connector. Anyway once you ground it you should get a solid horn.



The temp sensor provides the ground for the horn when the motor overheats. So if you ground the tan lead before the temp sensor you can test the horn. Understand your not grounding the temp sensor just the wire leading to it.
 

arch2

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Sep 4, 2012
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Re: 1999 70 hp johnson low rpm no power

I appreciate the feedback on this site. It has been very helpful. That being said...I'm still having frustrations with my 70 hp johnson motor. Over the past 2 weeks I took it out and ran it 2 different times with fresh gas and a can of seafoam in the tank. Both times it started and idled fine right off the trailer. Both times it jumped up on plane at full power and down the lake we ran. Once I ran about a mile before stopping to fish, and yesterday I ran about 15 miles at full power before shutting her down to do some fishing. In the first instance, when we fired the motor back up, it was the same old story...only 3200 RPM and 20mph max. The motor doesn't smoke or knock, just won't turn the RPM's it should at WOT. Yesterday, it ran great on the ride up the lake. We stopped to fish for 20 minutes or so, then re-cranked the motor and took off. HURRAY, we thought...finally full power after a stoppage and restart. Our celebration was short lived however, as about a half mile into our next run, the motor abruptly dropped to 3200 RPM and 20 mph, nearly unseating us. Again, no abnormal noises or smoke.

The intermittent nature of this problem has got me thinking electrical problem. A friend of mine suggested it could be the powerpack or the associated S.L.O.W. system kicking in. I suppose a bad temperature sensor or something could trigger the S.L.O.W. system, but I haven't gotten any alarms or lights on the dash indicating a problem. How much does it cost to replace a powerpack, and is it something I could tackle without taking it to the shop? Thanks for any help in advance!

bneely,

I am also having this exact same issue with my 1999 Johnson 70 HP and was wondering what you did to resolve your issue. The last time I had it out it started great and I was able to get 5500 RPM and 38 MPH, I then trolled for about 1.5 hours and when I went to leave I could only get 4200 RPM and about 25 MPH. Thanks for any input on what you did to fix this issue.
 
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