2006 Mercury 115 hp optimax - Carbon buildup on cyl #3 plug

[Cpartipilo]

Hello experts, I need your suggestions again.

I have a 2006 mercury optimax 115hp, serial 1B255833 i've owned since new.
This motor has run flawlessly all the time, and I have to admit maintenance has been ok but not what should have been to be perfect, but I started to put more love in it recently.

It recently started idling a bit rougher than normal, and when I took it to the water to do a trailer swap I decided to do a quick run to get fuel flowing thru, but it had very little power, and could not get over 2500 RPM. I did not get any smartcraft alerts or recorded any faults.
I began diagnosing by removing all the gas from the tank and adding new non-ethanol gas, emptied the VST, changed the VST filter, rebuilt the diaphragm pump, cleaned the VST (thing looked brand new inside), tested fuel pumps and all the normal fuel maintenance. I pulled the plugs to replace and noticed that the bottom plug electrode was completely covered in carbon, so most likely that's the reason for the loss of power as I doubt there was any spark there. Later I swapped the plugs and put new NGK plugs in it.

I figured the injectors could use service and a bad spray pattern could be the reason for the high carbon in the plug. Pulled the injectors and had them serviced by a local shop, and cleaned my fuel rail and hoses but barely any dirt came out, everything looked really clean.

Put everything back together and had the motor running up to temperature with the clean injectors. Idle is much better with a slight vibration which I think is normal but after about 20 minutes of running time I decided to check on the plugs and while top and mid looked new, the bottom plug looked like carbon is already building there.

I don't know if this is normal, or not. for the time being I swapped mid and bottom plugs to see if the new plug on the bottom begins building carbon too.


While I do my tests I would love to hear suggestions from the experts if there is anything I should be looking into or if I'm overthinking too much and this is normal.

[JOE54]

id run it a lil bit see if it cleans up could have been buildup in cyl ports or piston top

[Cpartipilo]

Thanks Joe, I got a can of power tune so i will do a good decarb cleanup before my next outing.
Also added quickleen and quckcare to my tank to help reduce that carbon buld.

[JOE54]

do not spray in intke

[JOE54]

do not spray in intke

[Cpartipilo]

Joe, thanks for the heads up. I was already aware but a reminder is never bad.

i’m planning to tilt up, spray it directly in the cylinders thru the spark plug hole and let it sit overnight. I saw a few posts where Don explained the procedure. It should help get the carbon lose for the quickleen to clean that up.

[JOE54]

[leckbass]

do not spray in intke

Why is that? I thought it should be sprayed in there?

[EuropeanAM]

Why is that? I thought it should be sprayed in there?

The only thing that belongs in the intake on an OPTIMAX model is AIR (and the oil delivered by the oil pump itself). No fuel is ever present there to "wash" away anything you would spray into the intake, or to carry it to the cylinders.

[leckbass]

The only thing that belongs in the intake on an OPTIMAX model is AIR (and the oil delivered by the oil pump itself). No fuel is ever present there to "wash" away anything you would spray into the intake, or to carry it to the cylinders.

Thank you for that info! Never knew that before. I've done it on many DI Evinrude motors over the years.

[leckbass]

The only thing that belongs in the intake on an OPTIMAX model is AIR (and the oil delivered by the oil pump itself). No fuel is ever present there to "wash" away anything you would spray into the intake, or to carry it to the cylinders.

Thank you for that info! Never knew that before. I've done it on many DI Evinrude motors over the years.

[Cpartipilo]

I want to revive this thread with the final fix of the issue for anyone in the future.

My loss of power issue ended up being a bad coil, which seem to be common with optimax units. replaced all 3 coils with some that are stamped 1690 - 8M0047456 - Rev C that look to be from recent production.
My old coil was cracked open in the end where the tower exists which is never a good sign.

The combination of a bad coil and a carboned up plug was giving the motor timing rotation issues that were also creating lower unit clanking noises especially at idle RPM. Now all I have left is to do the Power Tune cleanup since I bet there is all kind of gunk in the cylinders and exhaust after a few hours with bad ignition, and then run it like I stole it for a few hours!
Took me a while to handle this since I was replacing the steering spindle, motor mounts, etc. but the outboard seems to be in top shape now.

[EuropeanAM]

Don't use power tune with NEW spark plugs. It will coat the plugs and they'll have to be replaced.