Single Spark plug on V-6 Yamaha fouling-up

[carlitos]

The spark plug on the # 5 cylinder ( bottom left as viewed from the rear) - on my 2 stroke 2003 V150 HP TLRB s/n prefix 6J9 with about 335 hours on it, turns a blackish color and eventually fouls-up.

I would truly appreciate help from y'all on what else to check, or do, in an effort to resolve this problem.

So far this is what I have done - but I did not really find anything that was amiss.

1) ordered the corresponding OEM service manual for it's' specific model and serial number.
2) insured that the pilot screw on the respective carb was adjusted to spec.
3) Insured that the gap on the recently installed spark plugs were to spec.
4) Insured that the ignition spark gap was at spec.
5) tested compression on said cylinder and it was at 120 PSI - which was pretty much in line as the other 5 cylinders.

I use non-ethanol fuel and use the Yamaha Ring Free along with it.

I normally run between 3,700 and 4,000 RPM on the way out and in between fishing spots,, and then run in WOT for a bit on the way back in.

Thanks

[FabricGATOR]

Is this the first time and you just got a plug that failed or have you been chasing this fault for a while? what do you do when it has happened in the past? Did you try to move that spark plug to another know working cylinder and that the good plug from that cylinder and put ti in #5? Did the new, good plug g et fouled and the fouled plug in a good cylinder clean itself up? How many hours running before it fouls up again?

There is a section in the beginning of the FSM that describes the maintenance inspections to be performed annually, In there is a check for the resistance for the ignition wire spark plug cap. You could also try to move the cap with another of the other cylinders to see it the fault moves to that other cylinder...

Another thing might be to remove the silencer (intake cover) and with the engine running, I put my palm over just that one the carb opening and let the engine vacuum suck that carburetor circuit momentarily, like a choke. That, many times, will suck the carb jets clear of anything creating a blockage.

Let us know what you find and how you get it working. I love to hear results!

[carlitos]

I went to pull the plugs as part of my bi-yearly maintenance and notice that one of them was fouled-up - and so not thinking much of it at the time because of the time span on them, i just went ahead and replaced them all with the new set that I had already purchased. After I had put four hours on the boat fishing I once again pulled the plugs again and noticed that the one in #5 hole had turned black.

I just now went ahead and un-screwed the plug cap and the resistance on it it shows to 4.53. I also checked the plug cap on # 1 and that one reads 4.69.

After thinking about it here for a bit I guess that what I am going to do is switch a couple of plugs and their corresponding ignition coils - one set with the other just for good measure, and I am also going to go ahead and do the vacuum trick on the carb and see what happens.

My buddy and me fish the moons so we are going tomorrow on his boat because it is his turn and them it will be my turn on my boat on the 19th - so if all goes as planned I will then let you know whether or not problem was resolved

Thanks for the help I truly appreciate it.

[TownsendsFJR1300]

If it turns out your spark is fine in that cylinder, compression is normal, I think you'll be checking the float, needle and seat to that lower carb.

May very well have half that carb (feeds two cylinders/ two throttle plates, etc) leaking too much fuel into the cylinder, fouling the plug


https://www.boats.net/catalog/yamaha.../v150tlrb/fuel A fuel pump does bolt near the lowest carb however if it was leaking, I would think it'd foul both lower plugs (and the pump is closest to the RIGHT SIDE, not the left, fouling cylinder)

Carbs: https://www.boats.net/catalog/yamaha...lrb/carburetor

[zenoahphobic]

Swapping coils around, my bet.

[99yam40]

fuel pumps would be an easy thing to check on.
they run off of a crankcase pulse, and if leaking could dump too much fuel into a cylinder

[zenoahphobic]

Swapping coils around, my bet.

Remember don't do a lot all at once because you might miss what actually the cause was if it fixed the problem.

I say moving the coils to see whether the problem also moves is a good idea because the fuel system seems it should affect at least two cylinders and not one on it's own.
Plugs, leads as well as ignition coils are regarded as maintenance items that tend not to last the life of the engine.
Although it rare for a Yamaha coil to fail, I have seen one or two that created subtle changes, including poor starting but seemingly faultless running. The ignition system (high voltage) is hard to test away from the engine, even the most sophisticated contraptions often miss a failing coil. A bit of my bugbear, as I have seen enough of changing coils eliminating some symptoms, as not just as a "try throwing this part at it" hit and miss poor diagnostics.

[99yam40]

does not each cylinder have its own crankcase?
seems to me 2 cylinders sharing a crankcase that it's fuel and air charge come thru would be a problem.

[rodbolt17]

ok.
the pointy end is the bow.
the square end is the stern.
looking at the pointy end on your left is called port.
right is called starboard.
this NEVER changes no matter where you are looking or standing on the dock barking at the moon.
port is port,stbd is stbd bow is bow and stern is stern.
keeps the confusion down
port side of you motor from top to bottom is 2,4 and 6. stbd side is 1,3 and 5.
monitor CDI output to that cyl.
yes a blown pump diaphragm can create that issue. so can a bad crankcase check valve.

[carlitos]

Excellent and Priceless Feedback.

Other than checking the fuel pumps for leakage - which is minimally invasive procedure, I am going to leave it all alone, and as it is until I go out fishing again here in a couple of weeks.

Hopefully the "single spark plug " fouling problem will have been resolved - and if not, then I will have a look at the lower carb as well as the other suggestions.

Mr. rodbolt17, you are most certainly correct, it is cylinder #6 that is given me the problem - not #5, - as I had erroneously stated. I stand corrected Sir.
Would you please help me out and kindly help me understand how one would "monitor the CDI output to a Cylinder" , as well as the "crankcase check valve" ? I looked in the parts book and found a check valve 624-11379-00 but this one seems to sit in be cylinder block journal and as such seems difficult to get to.

Thanks Again.

[FabricGATOR]

Is the plug "Fouled" to the point where it is misfiring?
Or are you just being observant and finding a rich condition due to the spark plug presenting with a little black carbon?

I suppose I just assumed you were having a misfire based on the title "Single Spark plug on V-6 Yamaha fouling-up"

[zenoahphobic]

Is the plug "Fouled" to the point where it is misfiring?
Or are you just being observant and finding a rich condition due to the spark plug presenting with a little black carbon?

I suppose I just assumed you were having a misfire based on the title "Single Spark plug on V-6 Yamaha fouling-up"

Good point

[carlitos]

The set of spark plugs in question had 37 hour on them when I went to do my scheduled maintenance (water pump, drain and refill lower unit gear oil, check function of thermostats and poppit valve, replace spark plugs, etc.) and that is when I found that the insulator and electrode on #6 were completely covered with a heavy coat of carbon to the point that it could not have been able to fire effectively - as it is supposed to. The other five plugs were what one would normally expect to find with a sort of tan color on them and a clear distinction between the insulator and electrode.

In all honesty I have a hard time hearing an actual misfire but the motor ran a lot smoother after I replaced the plugs so I will say that it was misfiring.

Thank you so much for sharing your insight.

[carlitos]

I think that we may have identified the root cause of the problem.

I went ahead just just now and backed out the two mounting bolts for the lower fuel pump and squeezed the fuel primer bulb till it was full and then went and looked and the pump and sure enough there was fuel dripping out from behind it.

I then did the same procedure for the upper pump and there was no fuel leaking out from that one.

What a relief.

I will let y'all know how it all works out once i get the failed pump replaced and go out and run the boat.

Thanks

[FabricGATOR]

Good find.
Yes, please report back if that was indeed your cure and how you repaired it (change pump vs rebuild)
I believe you will likely find the complete pump is less expensive than the parts to rebuild it.