Yamaha 40hp 2 stroke rough idle

[edevanson]

Hi, I have a 2001 Yamaha 40hp 2 stroke that will not idle. The motor starts and idle very rough and then dies. The issues appears to be with the number one cylinder. If I disconnect the number one spark plug wire, the engine will stabilize and idle on two cylinders. If I reattach the number one plug wire the engine will sputter and die.
Also, a spark is visible between the number one coil and the engine, where the coil is attached. The plug wire seems charged along the length of the plug wire, as a slight sparking is present when the wire is touched.
I swapped out the coil, but the same issues as stated above remains, including the coil to engine block sparking.
Compression is good on all three cylinders.
Spark check good on all coils.
Swapped around spark plugs from cylinder 3 to 1, but no difference, same symptoms as above.

Any advice on what is causing this issue would be greatly appreciated.
Thanks

[throrope]

I struggled with a truck that ran rough just before pulling our travel trailer for a weekend trip. I had just replaced the plugs. As I struggled into the dimming dusk, the sunlight waned just enough to see an entire lighting storm throughout the engine. Apparently, the wires were on their last leg and the flexing from the plug change did them in.

I suggest running on muffs in evening and watch for arching. It may be only one issue. If you find a lightning storm, Break Out Another Thousand (BOAT).

Beyond that, I'd schlep it to the best Yamaha certified tech in the area, have 'em do a physical and let him put all in order. Sounds expensive until you buy parts you don't need, miss out on boating or even worse, replace a good motor.

[a387673]

I wonder if new plug wires and/or plugs would fix it?

[rodbolt17]

plugs, maybe.
that's where my Kv tester comes in handy.

if something,bad plug,lean fuel mix, is causing excessive voltage builup in the coil secondary that's is what happens.

[99yam40]

I understood a plug under compression took more voltage to jump the gap on plug, but did not know a lean fuel to air mix could cause a need for higher voltage.
as far as plugs go ,it is easy to change plugs or at least swap them from hole to hole to see if condition moves if you do not have new plugs handy

[edevanson]

Thanks for all the input.
OK, swapped the plugs around, as well as the coils/plug wires and the stray voltage "lighting storm" (good one throrope ) remains visible on the #1 cylinder plug wire and coil. Can probably rule out a bad coil/plug wire and spark plug.

After the coil and plug swapping, I still have a rough idle. When I unplug the #1 spark plug wire, the motor stabilizes and will idle on two cylinders.

So it looks like the stray voltage on the #1 cylinder coil and plug wire is causing the rough idle. Now the question is, what is causing the stray voltage?

rodbolt17, you mentioned excessive voltage buildup in the coil secondary. Can you explain what the coil secondary is?

Anyone have any other ideas on what could be causing the stray voltage?

Thanks

[throrope]

I haven't had the pleasure on an outboard, but as I understand, once an arc occurs, it leaves behind a carbon track straight through the Insulation or what ever the original arc jumped through that then becomes a highway for stray voltage and continued arcing. Electricity takes the path of least resistance and the higher the voltage, the bigger the jump. If the plug goes bad, it will find the next path of least resistance thereafter. Hence the reason wires require replacement and why I tossed the "original" plugs, which are the cheapest parts of the ignition system.

[rodbolt17]

you gotta remember, electricity,Bass and myself are simply lazy.
that 40 HP ign system is capeable of about 30 Kv.
it typically only sparks at about 7 Kv.
much more than that indicates an issue.
as the magnetic field collapses acrossed the secondary coil windings voltage will build until it can arc,someplace.
late model 4 stroke stuff with the coil on the plug typically spark above 32Kv.

[99yam40]

Just a though I had that I wanted to share even it may or maynot have anything to do with this motor.

It is a very bad idea to pull plug wires on a running motor to see how it reacts, as that causes the spark to find a different place to arc. It may be inside the coil or anywhere else it can find.
Always use a spark tester and make sure motor is off when unhooking and plugging plug wires on the spark plugs.

Kind of sounds like if you could measure the output voltage and it was above the 7K there is a problem you need to find

or if it is below the 7K but arcing to somewhere besides the plug or spark tester set to the proper testing gap there is a problem with the insulation on secondary side of coil to the spark plug

[zenoahphobic]

Anyone recommend on replacing Hightention wire? Or replacing the cap? (If only temporarily).I know it is moulded into coil and can't be conventionally tested.
It was suggested that in vehicle they go bad.
That the faulty cylinder ignition pulls down the other cylinders clearly indicates that the internal resistance of the faulty setup has significantly reduced.
You may have damaged you system pulling the leads physically, or when the engine was running.

[99yam40]

Anyone recommend on replacing Hightention wire? Or replacing the cap? (If only temporarily).I know it is moulded into coil and can't be conventionally tested.

He did say he swapped plugs and coils with other cylinders and still had the problem on same cylinder, so it is strange that the coil and plug wire would work fine on one cylinder but not on the other

I would test the pulser coils output voltages, along with charge coil,and CDI outputs to the coils. Also the idle timing along with the link and sync just to make sure all is in spec.
I do not see any of that causing the spark to jump out of the wires to the motor though
It is all easy to do with the proper equipment

[zenoahphobic]

He did say he swapped plugs and coils with other cylinders and still had the problem on same cylinder, so it is strange that the coil and plug wire would work fine on one cylinder but not on the other

I would test the pulser coils output voltages, along with charge coil,and CDI outputs to the coils. Also the idle timing along with the link and sync just to make sure all is in spec.
I do not see any of that causing the spark to jump out of the wires to the motor though
It is all easy to do with the proper equipment

If there is something that appears irregular and some analysis has been done and the mystery appears to deepen then clearly at this point we are clueless.
In other words trying to explain that something must work because it was swapped and it appeared to have worked elsewhere, is not really good logic.
If it added to the mystery this process of elimination simply does not eliminate the coils and everything associated with it. Eg has the earth contact between the coil and the engine been checked at the faulty cylinder?