[truckermt]

My 1985 yamaha 25 runs very rough under load and will not build rpm. At idle the engine is rough and eventually dies on its own. I had a similar problem before and after changing the plugs the problem went away. I have cleaned the carb and it appears to be getting fuel. Could the ignition electronics be going bad?

[media]http://s1109.photobucket.com/albums/h425/mttateo/?action=view&current=Video002.mp4[/media]

http://s1109.photobucket.com/albums/h425/mttateo/?action=view&current=Video002.mp4

 

[truckermt]

Forgot to mention I have replaced the plugs again as well.

 

[Brett]

You've got an 1985 outboard, and wondering if the ignition electronics might have gone bad?
Odds are they have. Even new outboards from Yamaha only have a 6 year warranty.
That might be a pretty good indicator of what their engineers expect the electronics lifespan to be.

 

[truckermt]

haha thats a very good point. Can you tell by the sound in the video if that could be the culprit though? Should i start by getting a spark tester and work my way up through the electronics?

 

[Brett]

If you plan to keep using it, you'd better learn how to use a volt/ohm meter
then test every circuit and compare to the specs in the Yamaha shop manual.


btw, I don't know enough to be able to diagnose by ear.
Only professionals can do that.
I'm just a barely competent amateur.
That pro staff under my name, that just means I try to avoid negative comments...

 

[twitch]

Sounds like it is running on one cylinder.

Were both plugs fouled or just one?

I'd start with a compression test, then a spark test....and work down the list of usual suspects.

 

[truckermt]

The bottom plug is the ashy one. Last time the motor acted up like this, around a month ago, the mechanic replaced the plugs and it ran fine. He did a compression test and both cylinders were 120.

 

[truckermt]

 

[twitch]

I'd look real close at the ignition components for that fouled cylinder/plug. Possibly a weak spark.

I'd also check your fuel pump. Unbolt it from the block and pump the primer ball, if any fuel comes out of the back of the pump it needs to be replaced or rebuilt. A faulty pump can flood a cylinder via the pulse port and foul the plug.

 

[truckermt]

Thank you i will check that out later today. When i remove the pump do i need to be careful pumping the primer or can i do it as usual.

 

[twitch]

just like you normally would.

 

[truckermt]

I unbolted the fuel pump and pumped the primer, there is not a leak there. Will one of those in line spark testers work to see if im getting sufficient spark at the cylinders, or do i need a different tester.

 

[Dadvocate]

http://www.sschapterpsa.com/ramblings/Johnson_9.9_troubleshooting.htm

This is page shows you how to buid your own with some basic supplies and you are able to compare spark as all cylinders fire in order.

 

[twitch]

As long as the gap is adjustable an inline should work.

I use a homemade tester similar to the one in the link.

what you are looking for is a nice blue spark that will jump about a  1/2" gap on the tester.

 

[truckermt]

I did a compression test and the top cylinder is right below 120 and the bottom cylinder with all the buildup on the spark plug was right below 60. I havent done the spark test yet because i figured the compression was pretty awful and most likely causing my problems. What is the next thing I should do. Is a rebuild my only option?

 

[cutrunner]

At this point , best case scenario would be you have bad reeds, other than that, yea u need a rebuild

 

[twitch]

At this point , best case scenario would be you have bad reeds, other than that, yea u need a rebuild

I agree. Although I might decarb the motor and recheck the compression.

 

[truckermt]

Would carbon buildup cause that much of a loss in compression though?

 

[twitch]

It could if a ring was sticking due to carbon build up.
If it's a broken ring or severely scored cylinder, the no, a decarb won't help.

 

[truckermt]

I ran a can of seafoam through and retested the compression. The cylinder that was at 60 is now back up to right below 120. The engine now will build rpm but still has slight hesitation at times and some stumble at various spots of throttle. I have hooked up a spark tester and I cant see a spark from either. I don't know if its because its too bright out to see, or I hooked something up wrong. I put the one end of the tester into the spark boot and the other end to a bolt that has a ground wire attached to it.