1991 Johnson 40hp Misfire when warm

[bradfr]

I have a 1991 Johnson 40 hp outboard that I am trying to troubleshoot. For some reason once the motor gets to warm operating temp it begins to sputter and shake really badly. Once this happens the motor will idle just fine and work for any RPM under 2500 or so. So far I have done the following: Compression check(good), replaced coils(old were cracked on bottom), cleaned carbs, ran fuel out of good fuel gas can. I am leaning towards a power pack issue but was looking for a way to test before I shell out $100+. Does this sound like your typical power pack issue? How can I test the power pack? Mine is located under the flywheel so testing during the hot condition might be a challenge.

 

[Rick.]

Re: 1991 Johnson 40hp Misfire when warm

What it sounds like to me is that your overheating/VRO has stopped working/oil tank level is low/ (one of these only) and the motor cuts your rpm back on purpose to try to save the engine. They say when this goes into that mode the engine does run rough. When you first turn your ignition key do you hear a chirp sound from the control box? Sounds like perhaps your chirper is not working as I believe it should be chirping at the same time it reduces the RPM. There is a proper name for the system but it escapes me right now. I'll have a look at my book later today. Mine is a 95 - 40HP. Johnson. Best of luck. Rick.

 

[bradfr]

Re: 1991 Johnson 40hp Misfire when warm

What it sounds like to me is that your overheating/VRO has stopped working/oil tank level is low/ (one of these only) and the motor cuts your rpm back on purpose to try to save the engine. They say when this goes into that mode the engine does run rough. When you first turn your ignition key do you hear a chirp sound from the control box? Sounds like perhaps your chirper is not working as I believe it should be chirping at the same time it reduces the RPM. There is a proper name for the system but it escapes me right now. I'll have a look at my book later today. Mine is a 95 - 40HP. Johnson. Best of luck. Rick.

Rick, this is very possible. Before all of this started happening we were getting warning signals for low fuel pressure. Once i started playing with stuff the alarm no longer came on. This is why i started with the carb cleaning thinking it was fuel related. The last time i turned the key on there was no chirp so you could be correct with the VRO chirper not working. I think the term your talking about is S.L.O.W.

 

[hidef]

Re: 1991 Johnson 40hp Misfire when warm

You are going into slow mode. Check the water pump and thermostat. Also find out why the warning horn isn't working. The alarm for your 40 will either be for overheating, low oil or no oil. The 40 doesn't monitor fuel restriction.

 

[Rick.]

Re: 1991 Johnson 40hp Misfire when warm

S.L.O.W. it is. My brain works the same way! Agree with above. You must trouble shoot the system as you could be hurting your engine and that audible alarm is your best friend. Get it working. I'm not even positive it is supposed to go off when your in SLOW mode but it should be sounding when you first turn the key. If you do a search here I'm sure someone has described exactly when and why the alarm goes off. Rick.

 

[bradfr]

Re: 1991 Johnson 40hp Misfire when warm

Well, I pulled the lower unit last night to check the impeller and sure enough one of the 3 ears was broken off. The other 2 were not torn all the way through but were on their way out as well. I am hoping the lack of flow by missing 1 ear as well as the ear still being in the impeller housing would cause the engine to overheat and the motor to go into slow mode. I do need to fix the alarm now as well though, that has stopped working.

 

[Rick.]

Re: 1991 Johnson 40hp Misfire when warm

Too bad about the impeller but at least you found a problem and fixed it. Hopefully, other than your warning horn, that's all the problem you have. Rick.

 

[bradfr]

Re: 1991 Johnson 40hp Misfire when warm

I guess i spoke too soon. I replaced the impeller and water tested tonight and the same symptoms are occurring. The alarm is in fact working as it does chirp when the key is turned on. The motor is not overheating at all because it is nice and cool to the touch when it acts up. It still takes about 10 mins of use wide open(4000 rpms) then it will die out and act like its running on 1 cylinder. Again when that happens it will idle and run perfectly fine at any RPM below 2500 or so. I am back on my thought that the power pack is bad. Does anyone agree with my symptoms and possible resolution?

 

[hidef]

Re: 1991 Johnson 40hp Misfire when warm

Check the temp sensor. At wide open throttle the thermostat is bypassed so if you are pumping water you shouldn't be getting the overheat alarm.

 

[bradfr]

Re: 1991 Johnson 40hp Misfire when warm

Check the temp sensor. At wide open throttle the thermostat is bypassed so if you are pumping water you shouldn't be getting the overheat alarm.

There are no alarms sounding anymore. I tried unplugging the temp sensor when it was acting up like this. I also touched the motor and it was cool to the touch. I do not think this is overheating.

 

[hidef]

Re: 1991 Johnson 40hp Misfire when warm

Have you tried disconnecting the rectifier when it starts to act up? Take a look at the CDI Guide page 39 and 40.

 

[bradfr]

Re: 1991 Johnson 40hp Misfire when warm

Have you tried disconnecting the rectifier when it starts to act up? Take a look at the CDI Guide page 39 and 40.

I have not tried that yet, I am also going to test the orange wires from the power pack for 150V when it acts up as well. Thanks for the tips.

 

[bradfr]

Re: 1991 Johnson 40hp Misfire when warm

One other thing that we have noticed is that the battery voltage according to the meter is only about 12 - 12.5 volts during full throttle operation. I am not sure if this could be the sign that something is going bad like the rectifier and/or stator as well that is contributing to other issues.

 

[hidef]

Re: 1991 Johnson 40hp Misfire when warm

It sounds like your voltage regulator / rectifier has gone south.