Intermittent power loss

I've got a 1990 Yamaha 40sd-jd I'm losing spark to my number 2 cylinder intermittently carbs are good fees rebuild checked fuel pump with vacuum gauge all good checking spark with a spark tester it comes and goes no rhyme or reason it will always be ok on initial start up then next time it will drop out then it will be ok. Tried swapping coils around no change and just put new plugs in it no change from either any other ideas on what to look at I have a manual but electronics aren't my strong point and it's looking to me like either the cdi or something else electric

I wish I could help you. Yamaha's are not my strong point, but a motor is a motor.
My first thought was a coil pack, but you said you already swapped them around. Perhaps a module/sensor?

We just went through this with a 1988 70 hp Merc. Come to find out, it was the key switch going bad. That's not to say your key switch is going bad. But, before you do like we did and start buying parts to see if perhaps "that would solve the problem....$$$ later), I'd have an outboard mechanic take a look at it and diagnose it. All you are out is a service charge. (We paid $75 for the service charge, $25 for the key switch and installed it ourselves.)

Also, just a thought, if the oil sensor for low oil is out, it won't let you rev up the rpm's. That has happened to us on another motor we had. :idea:

Someone on here will be along to help you out. I'm sorry I couldn't be of much help. Good luck!

M
Tyler, Texas

Weird where you say it is always #2 cylinder that drops out, even after you switched coils around. So ... what root problem could be common to cyl #2?

Dumb question - but does that motor 'kill' fire to a cylinder when shifting from Neutral to Forward? More so it is used, usually on bigger motors, to help unlock the LU gears so she doesn't stay 'locked' in gear, due to torque.

But IF that OB did this ... and it was tied into cyl #2 ... then maybe that switch is intermittent or kaflooey :?: ?

I know it's a bit out of the norm, but you might also want to check/replace the plug wires. It wasn't on my OB but I actually had a plug wire once that would cause intermittent loss of spark as the wire was heated up by the running engine. Similar to the way a coil can fail when it gets hot.

I checked the low oil switch per the manual and swapped coils around to no help so I don't think the wires have anything do with it all it a pull start so no key and only kill switch is good but I think that would allow no power at all not just one cylinder. Also for got in first post compression is good all cylinder within 3 psi of 115 and 120 is is a new motor spec

Hang in there. These guys are smart on here and will share their knowledge. I felt certain they would come along and help trouble shoot it. Great bunch of guys on here.

So you haven't noticed the failure coincide with anything else, like only after the engine has warmed up or only when it's cold, or only after it's been idling for prolonged periods, etc?

I'm wondering if that motor has some kind of temp sensor that limits RPM if its overheating. They rarely go bad but........ also if you have DVM ohm out your plug wires , won't cost you a dime . I did once have a faulty plug wire that almost drove me insane. Just a shot in the dark.

It has to be an electrical control unit

My first guess would be plug wires. Bad plug wires can be fine one moment and then arcing off of the motor somewhere killing the fire to the cylinder. All it takes is a small pinhole in the insulation smaller than the eye can see to cause problems. They can also get loose connections on the ends losing the ability to keep the spark alive in the cylinder.

It does have an overheat switch but it's connected to a warning light which has never come on. It will always run good on initial startup then it comes and goes with no reason that's why it so hard to figure out cause it only does it when it wants. I thought coils or wires to but I tried swapping coils and it's always same cylinder that looses spark which leads me to believe it's not the coils or wires I'm waiting for a multi meter I ordered to arrive then I'm going to check the coils and whatever else I can just not very good at this electrical stuff

You swapped the coils. Did you swap the plug wires? Another thing I have come across in my years as a mechanic is bad spark plug wire boots. I have seen the rubber boots fail on motors before. They will get soft an no longer seal well enough to keep the wires from shorting out. I have seen the wires come loose from the crimped-on ends inside the boot also. Checking spark plug wires with a multi-meter is a little more involved than checking continuity in a copper wire. You have to take into account the composition of the wire, the length of the wire and any other variable for that specific wire to know what the reading should be to be sure you are getting the correct resistance in the wire. Not all wires are the same.

Test run after dark with the engine cover off. If you have any spark leak[ short] you will see it. Just don't touch it! Hank PS. This will only show external shorts, you cant see it if its internal.

I'll try it after dark didn't think of that. But when I swapped coils I left the plug wires attached to coil it has three separate coils with real short wires to each tried them in all different configurations even took the ends off and cut them down and reattached them Ends are weird not actually crimped it's like a spring that you just stab through the wire seems stupid to me but its the way they are

Well I figured my problem the ground wire for low oil switch was starting to break at connector must have been losing contact intermittently causing my problem had it out yesterday to test and it was fine. It limiting rpm to 2000 not sure why I was loosing spark but everything's good now thanks for everyone's help. Just have to figure out why my warning lights not coming on gonna test the light on my battery and see if it's bad

Glad to hear you have it figured out. Bad grounds can cause you all kinds of headaches. They can cause problems that do not seem possible.

=D> =D> =D> Your update just made my day!

It limits RPM by intermittently cutting spark to one or more cylinders. If you turn the key on and off really fast, the motor sounds almost the same as the RPM limiter function-because, well, it does the same thing more or less.