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Old 17-04-2016, 08:39   #1
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Johnson 4 HP Outboard Low RPM

I'm hoping someone can help me troubleshoot the issue with my 1986 Johnson 4 HP Model No. J4BRHCDE. This is the issue:

Motor starts and idles fine. If throttle is increased while not in gear, sounds like the motor revs up fine, but donít know actual RPMs so canít know for sure if it is really at max RPM. When in gear, and throttle is increased (it is a slide on the front of the motor housing, not a twist on the tiller) the motor rpm comes up, but does not increase as much as it should for where the throttle is. For example, when it is full throttle it is only about half the rpm used to run. Motor seems to run fine and I donít notice any unusual vibration, it just runs at a lower rpm than it should. Then, from time to time, while I am running along at full throttle/half rpm, the motor will suddenly go to full rpm. It sounds to me like there is a ďclunkĒ in the motor at the same time (and every time) it goes to full rpm. The motor will then run fine at full rpm until next time you throttle down. Then, you are back to full throttle/half rpm situation.

I suspected it was a throttle linkage problem. It has a cam/roller wheel assembly, and when I pulled the cover and inspected it the roller wheel is a little wobbly but it looks like it is always bringing the carb to full throttle, so now Iím wondering if it is a carb problem (something stuck in there is obstructing fuel flow almost all of the time) or an ignition problem (only running on one cylinder most of the time, but occasionally the cylinder fires).


Anybody ever had this happen?
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Old 17-04-2016, 08:48   #2
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Re: Johnson 4 HP Outboard Low RPM

What about the little diaphragm fuel pump? They get stiff over time and stick. It may be that it is only oscillating enough to feed half power, then it "breaks over" if you will, and moves the full amount of fuel.

At the very least it's cheap and easy and probably due for service anyway.


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Old 17-04-2016, 09:20   #3
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Re: Johnson 4 HP Outboard Low RPM

Thanks Sailmonkey, I had not thought of that one. Is that the symptom for a stiff diaphragm, i.e., low fuel pressure?
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Old 17-04-2016, 10:15   #4
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Re: Johnson 4 HP Outboard Low RPM

Quote:
Originally Posted by butch View Post
Thanks Sailmonkey, I had not thought of that one. Is that the symptom for a stiff diaphragm, i.e., low fuel pressure?

Not guaranteed, but it can be


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Old 17-04-2016, 15:44   #5
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Re: Johnson 4 HP Outboard Low RPM

I've worked on that model a couple times for our local club. Have you checked the fuel mixture knob? The throttle is on the top and the choke is on the left and fuel mixture on the right. Is that correct? Once in awhile the one I worked on would vibrate enough to change fuel mixtures on its own. I believe I closed it by turning it all the clockwise and then 1 and one half turns counter-clockwise to start. As the engine warms then turn it clockwise to lean the mixture until is sounds right. Too far and the engine will stop.

I'm hoping my memory serves me correctly about that engine. I remember the cover being difficult to remove to check other things but taking off the carb and flushing and cleaning never hurts. Check the fuel shut off valve for debris. Sailmonkey might be right too about the diaphragm.

Good luck in your troubleshooting.
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Old 17-04-2016, 15:49   #6
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Re: Johnson 4 HP Outboard Low RPM

I had that problem with a Yamaha. It was one of the main bearings. Diagnosed by OB mechanic who was cruising and solved it for me Trinidad. He and I changed out the main bearing in about 1.5 hours.
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Old 17-04-2016, 17:53   #7
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Re: Johnson 4 HP Outboard Low RPM

I ran it today on the bench and when I pulled one of the plug wires it died. So, it has been running (pretty good) on one cylinder, and occasionally it will get fire and hit on both cylinders. So, start with plugs and work back from there to plug wire and then coil.

Skipr John you are correct the cover is a hassle, and part of it is bolted on.

Thanks to everyone for your help.
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Old 17-04-2016, 18:13   #8
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Re: Johnson 4 HP Outboard Low RPM

If this is a two cylinder engine, as you imply, it sounds suspiciously like an intermittant shorting of a spark coil, commonly called a "power pack."
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Old 17-04-2016, 18:30   #9
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Re: Johnson 4 HP Outboard Low RPM

2 cylinder 4 HP?


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Old 17-04-2016, 18:33   #10
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Re: Johnson 4 HP Outboard Low RPM

Looking up the parts for it, it does indeed seem to be a two cylinder engine, but with one ignition coil.
I'd replace the plugs and see what happens, or at least swap plugs to see if it then will run on the other cylinder.


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