Topic: 1974 Johnson 15 hp runs on 1 cyl at low speed

This topic has 19 replies, 8 voices, and was last updated 8 years, 8 months ago by Mumbles.

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October 6, 2016 at 12:03 am #5434
timmy3131
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I have a 1974 Johnson 15 hp on my duck boat. It seems to run in 1 cyl at low RPM. If i get above a certain speed it runs like it should. Does anyone have any ideas?
October 6, 2016 at 1:22 am #45277
Steve A W
Participant
A good place to start is the spark plugs. Replace them or swap them
hole for hole and see if the problem follows the suspect plug.
Next is a compression check. If you don’t have a compression gauge, you can barrow
one from a auto parts store.
Don’t worry about how high the reading is, but how close the cylinders are to each other.
Next check the ignition. Someone else will have to help with this as I’ve never worked
on a motor this new. I’m not sure if it has points or solid state ignition.
Good luck with it.

Steve A W

Member of the MOB chapter.
I live in Northwest Indiana
October 6, 2016 at 1:42 am #45280
garry-in-michigan
Participant
Lifetime Member
Check to be sure one of the shorting switch wires is not shorting out at slow breaker plate positions. Look for a slipped insolating sleeve or worn insolation. . . 😉
October 6, 2016 at 3:53 am #45291
PugetSoundBoater
Participant
The 1974 -1976 9.9 and 15 hp have points . The"Fireflash" low tension ignition system can be a bit troublesome,plugs can foul easily. Champ UL81 C gapped @.030 is what the plugs are supposed to be, but i have used NGK B7HS with success. The Champions are hard to find, Marine Engine.com has them.Need the model # for the correct plugs. How long has it been since a complete ignition tune up? If you remove the flywheel you will be able to check the points and test the wiring with an ohmmeter. Timing is critical. As Garry stated,visually inspect the wiring coming out from under the flywheel they flex and move as you move the throttle and the insulation can wear or crack ,causing a short to ground.EDIT

"Some people want to know how a watch works, others just want to know what time it is"
Robbie Robertson
October 6, 2016 at 3:59 am #45292
PugetSoundBoater
Participant
What is your model # on the metal tag riveted to the side of the transom bracket?,If that tag is missing the powerhead has it stamped on a aluminum plug on top of the powerhead near the cylinder head.

"Some people want to know how a watch works, others just want to know what time it is"
Robbie Robertson
October 6, 2016 at 12:26 pm #45305
timmy3131
Participant
The model is a 15R74S. I traded a Chrysler 9.9 for it that I had, the guy sent me a video of it running, which it does and when it’s wide open it runs great! I haven’t gone through it at all, so it could need points etc. I’m running NGK plugs in it. I noticed one of the coils was cracked but working when I got it so I purchased an OEM Replacement. It would make sense that there is a short somewhere because of the way it runs at a certain throttle postion. IF I were to run it in the dark and take the top cover off, if it was that problem would I be able to see it arc?
October 6, 2016 at 1:28 pm #45306
fleetwin
Participant
US Member
Have you done a cranking spark check on the engine? It should be able to jump a 1/4" gap on each cylinder while cranking. Don’t worry too much about the cracked external coil, this is not uncommon and does not necessarily mean the coil is bad. Garry makes a good point about the primary leads coming from under the flywheel, these can get frayed, split, shorted to the mag plate easily. You may see/hear an arcing coil more easily by running in the dark, but shorted primary leads probably won’t be visible.
For now, let’s start at the beginning, spark and compression test.
October 6, 2016 at 6:21 pm #45317
PugetSoundBoater
Participant
The correct champion plugs shown for your model 15R74S are L82C or the hotter L86C

"Some people want to know how a watch works, others just want to know what time it is"
Robbie Robertson
October 6, 2016 at 7:37 pm #45319
timmy3131
Participant
Thanks for the help!
I will check compression tonight when I get home and do a spark test also. I’m running the NGK bz7hs-10 plugs in it now.
October 6, 2016 at 11:54 pm #45342
retiredoz
Participant
The plugs you’re running now are for ”77 and later with CDI . I’d try the L82s..
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