Carbon Fouling Plugs

[NicksterFL]

Hi Folks!
Engine: Mercruiser 3L

So It seems I keep carbon fouling my plugs when idling a few times on hose/muffs.
Is this because the engine isn't under load and hasn't burned off the carbon?

Another forum suggested i was running rich. I checked my idle screws (Rochester 2GC carb) and they were only turned out 1/2 turn each, however, the idle sounds great and she idles around 850-900 RPM's on hose (lower in water).

I tried readjusting by turning out 1.5 turns. set RPM's to 800 then turned 1 screw in until it stumbles, well it turned all the way in (seated) before it stumbled. Then I did the other. Ended up 1/2 turn out again and then readjusted RPM.

thoughts?

 

[Bt Doctur]

try adjusting the choke plate to open sooner

 

[NicksterFL]

choke is completely open when cold.

 

[Scott Danforth]

If your choke is open when cold, and you can turn the idle screws in all the way, I think you need a carb rebuild

 

[NicksterFL]

Odd thing is this is a new remanufactured carb.

I did notice there is a very slight dribble of fuel from the holes that the accelerator pump would squirrel out of (or at least that area).

Is this a float level issue?

 

[achris]

May not be a fuel problem. Weak spark will cause the same thing, carbon fouling. First question, dry foul or wet? Wet is oily, and could indicate rings, valve guides etc. Dry fouling (just sooty), leads to the next question, which ignition system do you have on your 3 litre engine? (3l has been running since 1969, and has used a variety of ignition systems, points, DDIS, EST). Then check the plugs are the right ones, check the leads (including the coil to dissy cap lead), distributor cap and rotor are in good condition. Then depending on which system you have there are other checks to do.

Chris........

 

[NicksterFL]

Dry carbon fouling.

I put in the flamethrower 2 electronic ignition and matching coil. I also bypassed the resistance wire to the module. Distributor cap and wires are new.

Again it only seems to do this when I run the boat on hose a few times and the plugs are black.

This is also an older engine (1971) and compression is around 115 each cylinder.

 

[NicksterFL]

Spark plug i'm using is: MR43LTS

Pertronix Ignitor II
Pertronix Flame-Thrower 0.6 ohm Coil (resistance wire bypassed).

 

[Bondo]

Ayuh,.... That's the wrong plugs for that vintage motor,....

Oughta be MR43T,....
The ones ya got are too long, 'n are for the newer Vortec heads,....

Also, at 115 psi, she's gettin' pretty darn tried,.....

 

[NicksterFL]

thanks bondo.
I'm also running a 140 thermostat. is that too cool?

She's a tired boat, but I only take her out to fish, not wake boarding or anything.

Will wrong spark plug cause carbon build up like that? just idling?

 

[NicksterFL]

Ayuh,.... That's the wrong plugs for that vintage motor,....

Oughta be MR43T,....
The ones ya got are too long, 'n are for the newer Vortec heads,....

Also, at 115 psi, she's gettin' pretty darn tried,.....

Well, looks like I have a newer head on this engine. The MR43T's are too short and do not even reach the threads.

 

[NicksterFL]

maybe I should bump up to MRr46lts (hotter plug)?
I rarely run WOT. usually cruise about 70-80% throttle

 

[Bondo]

The MR43T's are too short and do not even reach the threads.

Huh,..??..??..??

We're talkin' 'bout the plug protrusion into the combustion chamber,.....
Not how easily they start into the threads outside the head,....
(edit; use a short piece of rubber hose/ fuel line pushed onto the outside tip of the plug, so you can spin it in)

I'm sorta surprised the pistons ain't wackin' the longer plugs,....

There's a castin' # under the valve cover to Id the head, but ya gotta pull the cover to see it,.....

I'd go back to the dribblin' fuel yer seein' in the carb myself, 1st,....

 

[NicksterFL]

yeah, i think this engine has different heads. The head has this stamped on the side: 488D11 ... The MR43T plugs do not reach the threads in the head. I took a photo of the sparkplug hole, and there's some distance first before the treads start.

photo: https://drive.google.com/file/d/0BxNoIFq0c64bdHZnRlplQnAyRjA/view?usp=sharing

I did adjust the float and fixed the dribble

photo of plugs (they all look the same after installing new, and running on the hose a couple times.

photo: https://drive.google.com/file/d/0BxNoIFq0c64bb3p4VmxhR2NGSHM/view?usp=sharing

My real question though is am I chasing my tail with this carbon build up, if that is "common" for running the boat on a hose? will it build carbon because it's not under load and the engine isn't real hot?

 

[Bondo]

Ayuh,... The threads in the head, look stripped to me,.....

My real question though is am I chasing my tail with this carbon build up, if that is "common" for running the boat on a hose? will it build carbon because it's not under load and the engine isn't real hot?

Generally speakin', No,.... yer results may vary,...

If ya got a rich condition, it may well reveal itself at idle, no load, no heat,...
More so than a rich condition at Wot, in the water,...

 

[NicksterFL]

ok. one last question.
Since i have a 'tired' engine with compression getting low (115's dry, 122ish wet/warm), should I consider bumping my spark plug heat range up to R44LTS or R45LTS plugs?

I don't run 100% WOT. I troll, and putt around the bay, she's an old boat... usually cruise about 75-80% throttle to get to where I want to fish.

 

[bruceb58]

You should have a sticker on your engine saying which plugs to use. Some 3.0L engines used the longer reach plug.

The problem with using a hotter plug is that it will not work properly when you are not at idle. You need to correct your rich idle issue.

 

[NicksterFL]

Happy to report back that my dribbling carb was the culprit. I readjusted the float. Went out today and trolled for 4 hours checking the plugs about every hour and they were a nice light tan/white color.