Looking for a read on my plugs, picture included

[hotrod53]

I have a '77 Merc 500 Thunderbolt. I put new plugs in this motor when I built the boat 4 years ago, I've had them out to winterize, but apparently didn't pay much attention. I'm running NGK center fires BUHW-2 and the motor has been running trouble free. Last winter I bought new plugs and today I decided to put them in. It has been starting well, but I noticed not quite as easy this year.

When I pulled the plugs, here is what I found, and these numbers are the percent of the center electrode remaining.
#1, 100%
#2, 80%
#3, 40%
#4, 65%

I'm familiar with 2 stroke engines through my dirt bike days, but outboards, not as much.
1, how often should I be changing plugs?
2. Is the loss of center electrode normal based on the 3-4 years in the water with an average of 6-8 outings
3. How do you explain the difference in plug wear from cylinder to cylinder?
4. Do you suppose any issues based on the electrode wear?

The plugs pictured are cylinders 1-4, left to right.

 

[MH Hawker]

i change mine at 100 hours

 

[Texasmark]

I don't recall ever seeing center electrode wear on surface gaps, Champion or NGK. I had a I6 115 that I ran for 7 years on 2 sets of plugs and I never saw center electrode erosion; track marks yes, but did not completely cover the ceramic insulator. From the looks of your insulator, seems those are seriously due for replacement. But that's my experience.

 

[gm280]

Sure looks worn to me. I would certainly change them out, but maybe take an initial picture and every now and then remove them for comparison. I ran surface plug in my Inline 6 115 HP Mercury for years without any problems. I would remove them and clean them yearly though. But they never ever looked that bad or worn and sooty. JMHO

 

[hotrod53]

I agree long overdue to change, and I did replace them. I remove them at the end if the season to winterize but I don't remember them looking that worn in the past. It's also odd that they aren't worn evenly. I'll bet that there is about 100 hours total on them, I did do a big trip last year for a week. I also rarely have no throttle position other than idle or wide open. I've been doing a 6 mile wide open run just about every outing to start.

 

[boltonranger]

Well... I don't have experience with center fire plugs but still, possibly a cooler plug is needed? I'm thinking heat is possibly a factor?

 

[Texasmark]

Faztbullet probably has the answer as to why Merc and at least OMC runs surface gaps; I knew once but forgot. There is a reason and they have been doing it from the factory since 1988 that I know of. The manual has an alternate conventional NGk for certain conditions. Bullet probably knows the answer to that too.

 

[Dukedog]

surface gap has been around for a very long time .. in fact, back in tha 60's, there was a big court deal between brunswick and champion over patent rights.. it was mainly developed for tha two stroke (gas n oil mix, fires twice what 4 stroke does), its [FONT=&quot]multidirectional gap (360*) for merc's (then new) "high voltage ignition"... think champion was tha first (L 19V).

all (2,4,6) should "wear" 'bout tha same.... should last a long time if tha motor is healthy...
tha premature electrode wear on one or two is something usually in tha ignition causing a higher temp with that plug.. could be a timing and/or switch box issue and may not be detectable jus by tha way tha motor runs... you might wanna "index" tha flywheel and check timin' on each cylinder if it concerns ya... but with tha way ya use it and been more or less trouble free, new plugs once a year and run it....[/FONT]

 

[gm280]

Before replacing a surface gap plug with a standard electrode type plug, make absolutely sure you have enough piston clearance so you do't hit the electrode. I personally would just buy the same and replace. JMHO

 

[hotrod53]

This motor had NGK conventional plugs in it when I got it, I also had a bunch of new spares in the boat. The previous owner said "never go anywhere without spare plugs". Once I put the surface gap plugs in, I've never had an issue or had to change them. I guess my biggest concern is that if the electrodes are burning, are the tops of the Pistons burning?

 

[mercster]

Mercury had issues with champion / electrode plugs not firing well with the thunderbolt system. Only use ngk's surface gap plugs and change them at the beginning of every season. Cheap enough on ebay.

 

[DavidMoore]

This maybe tosh but I'll say it anyway.

The only time I ever saw something similar was on a big diesel engine (about the size of a bus) designed to run on crude oil, running on crude means the fuel injectors had to be cooled.

Problem there was you start and stop a crude engine on normal diesel fuel (otherwise to fuel will solidify in the lines and you have a major headache). If the cooling system for the injectors was running when operating on diesel then the tips of the injectors would over cool, attracting moisture which went acidic when mixed with combustion products, the tips would then literally erode away and fairly quickly too.
I'm wondering if something similar is happening here?

So I say change them for the ones NGK ones that mercury recommend and don't go gambling with hotter / colder ones, and also check your not running rich or timing is too far retarded.

Maybe you could you mail the photo's to NGK and ask what they think? I for one would be fascinated to know what they say.

 

[Dukedog]

some folks jus seem ta "know better than Merc" (better plugs etc.) for some reason but think Merc has been around tha block a few more times than these same folks.... doubt your hurtin' tha piston tops considerin' tha age on tha plugs and no failure yet!.. a weak piston won't stay around this world very long.. one thing, if ya can index tha flywheel and check each cylinder's timin' then set max timin' (what ever book calls for) on tha cylinder that reads tha highest might put ya little more at ease....