1987 Evinrude 25hp

[Whoopbass]

The motor runs fine at wot and idle but coughs at trolling speed. I don't troll but if i'm going through a 5mph zone the motor idles...coughs....idles....coughs, etc.
I have owned this motor a few years and it has always done this. I cleaned out the carb when I first got it and no change.
I started fiddling around with it today and noticed after running it that the top spark plug is wet while the bottom one is dry and ashy. I have ran it then checked the plugs several times and it's always this way. I put a spark tester on it and they both seem to fire normal.

While running the motor at the RPM's when it coughs I sprayed some fuel mix directly into the carb and it smooths out so it appears it's not getting enough gas. I have turned the low speed adjustment in and out at various stages and it doesn't seem to affect the cough.

The compression for both cylinders is 110 lbs apiece.

I am going to thoroughly clean the carb shortly but I was wondering if the bottom cylinder plug being dry and ashy is normal.
The outboard runs fine otherwise.

 

[jasper60103]

Re: 1987 Evinrude 25hp

Maybe plugs are suspect? What plugs are you using? Try swapping plugs and see if symptoms follow.

 

[Whoopbass]

Re: 1987 Evinrude 25hp

Maybe plugs are suspect? What plugs are you using? Try swapping plugs and see if symptoms follow.

I forgot to mention that I just installed a correct set of Champions with no change in the way it runs. I also ran a can of sea foam through it before the new plugs. I also swapped coils with no success.

 

[Whoopbass]

Re: 1987 Evinrude 25hp

I think the cough might have something to do with the cold start primer injector. I can pull the knob out part way and it seems to smooth out. I completely disabled it earlier but that didn't have any effect. I have a parts motor with a primer injector on it that I will swap over. I don't know if this will fix the sooty plug but hopefully it will get rid of the cough.

 

[jasper60103]

Re: 1987 Evinrude 25hp

I think the cough might have something to do with the cold start primer injector. I can pull the knob out part way and it seems to smooth out. I completely disabled it earlier but that didn't have any effect. I have a parts motor with a primer injector on it that I will swap over. I don't know if this will fix the sooty plug but hopefully it will get rid of the cough.

Whoopbass,
it sounds like the primer is doing it's job.
By pulling the primer knob, you're feeding it more fuel to compensate the lean condition.
That's why it useful for cold starting.
If you pull the primer knob all the way out when the engine is warm, it will bog down.
At least mine does.

I think it may be sucking too much air.
Another test is to cover the carb opening with a rag and see if the sneeze will stop.
Else, if not sucking too much air, then it's back to checking the idle circuit for a fuel restriction.

Just some thoughts. Please keep us posted. Thanks.

edit: Another thought. Is the carb butterfly plate closed during idle? It should be to restrict air flow.

 

[Whoopbass]

Re: 1987 Evinrude 25hp

I have to pump my primer knob for it to deliver fuel to the carb. Maybe leaving it open will allow the engine to pull extra fuel into the carb. Who knows, but I know it did run better at the lake when I would pull it out some.
I switched primer injectors with the other one I had and it ran great for 1-2 minutes and then ran worse then before.
I think I have an air leak somewhere. I will have to check tomorrow since I ran out of daylight.

I will check the carb plate tomorrow. When it was idling low I remember it being closed all the way but the motor idles at low speed just fine.
I will try the rag trick tomorrow as well.
Thanks

 

[Whoopbass]

Re: 1987 Evinrude 25hp

I tinkered with it with no success so I went and pulled a 30hp carb off a parts motor I had. I got it cleaned up and installed it. Ran it on the muffs and it seemed to run good so I mounted it back on the boat and went down to the river. The outboard ran great. It seemed to run a bit faster but more importantly it wasn't coughing at idle speed. I idled it for a mile and it ran like a swiss watch. It appears the old carb was the culprit all along.

 

[jasper60103]

Re: 1987 Evinrude 25hp

I tinkered with it with no success so I went and pulled a 30hp carb off a parts motor I had. I got it cleaned up and installed it. Ran it on the muffs and it seemed to run good so I mounted it back on the boat and went down to the river. The outboard ran great. It seemed to run a bit faster but more importantly it wasn't coughing at idle speed. I idled it for a mile and it ran like a swiss watch. It appears the old carb was the culprit all along.

Good to hear you got it running right. Thanks for the update.