Crud and black plugs. Pics below

Ramanonos

Seaman Apprentice
Joined
Aug 15, 2011
Messages
32
Been having a whole bunch of issues with a new refurbed Mercruiser 357 Alpha 5.7L. Engine has less than 20hrs on it. I changed the break in oil at about 15hrs. I posted on the forum because I seemed to be unusally low on oil and had a lot of steam exhaust. Example: add a quart after a few uses. The forum said it's normal given that Im still in break-in mode an piston rings need to set (so this in turn is causing oil consumption and a lot of steam exhaust). Anyways, this might be important info because the engine has run rough the last 5 hours. Seems fine at 3K rpm, but pretty rough at idle. Also been taking a good 10-20secs to start if I've run her and then stop for an hour to fish. Starts up quickly if its cold and been sitting for days. So I decided I'd check the plugs. Here's what they looked like... Half were black soot, and the other half had a black caked on layer... Yikes. I replaced all 8plugs but Im nervous there's a bigger problem going on here. The engine and plugs were new (<20hrs). Should I not take her out? Thoughts? See pics below. Thanks.
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Ramanonos

Seaman Apprentice
Joined
Aug 15, 2011
Messages
32
Re: Crud and black plugs. Pics below

Oh, and there seemed to be some oil around the threads too when I took them off. No excessive, but it was there (threads only).
 

81 Checkmate

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Jul 20, 2010
Messages
1,360
Re: Crud and black plugs. Pics below

No expert... but looks like carbon build up from being to rich on the carb and from low RPM runs...Other words not blowing out the crud out the exhaust from running low RPMS during break in. Carb may be set right..it's just how the motor has been lugged.

What has been your max RMP runs during break in?

Did they/you put new valve seals in the heads?

If they were white then i would be worried...Meaning way to lean
 

haulnazz15

Captain
Joined
Mar 9, 2009
Messages
3,720
Re: Crud and black plugs. Pics below

I think the one on the right looks like oil sludge built up, not just rich carb jetting. I can't imagine having to add a quart of oil every few uses, that's excessive in my book. Sure, the engine will burn some oil until the rings set, but that happens in the first 5 hours of running the engine, it shouldn't continue to be an issue. The one on the left I wouldn't be overly concerned with, but it does appear that it's a little rich on the carb, or some light oil burning causing the carbon build-up. I'd be concentrating on the oil consumption issue for sure, with the primary culprit being whatever bank of cylinders had the plug on the right in your pic.
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
71,110
Re: Crud and black plugs. Pics below

Ayuh,..... What carb,..?? probably needs a rebuild... Check the contents of the fuel filter...

A compression test 'n leak-down tests are in order, to find the oil issue...
Just to see what's broke-in, or broken...
 
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