Topic: Spark plug: Old age or too hot?

[Fritz]

Hi,

last weekend, I've ran my 550 on the Autobahn (was late getting to a wedding) for about 100 miles one way. I've done 95-100mph most of the time. Top speed was 108mph on the stock speedo.
On the way back it only made 100mph for about 15 seconds and then felt like it was getting low on fuel. Slowing down to 90mph let the engine recover.

Today, I had the chance to inspect the spark plugs and found that #1 had a real short center electrode. Below are pics of spark plugs #1, #2 and a closeup of #1.

I have no idea what happened to the plug. To me it does not look like it melt, although those tiny spots on the insulator look like some overheating. The ground electrode does not look like it took any damage.

The sparks are NGK D7EA, coils are stock and caps have been replaced two years ago.
I've done a 3000 mile tuneup four weeks ago including carb sync.
The points look good and I didn't need to re-adjust the gaps.
I'm running the stock airbox, 069A carbs with stock setting and Motad 4-1 exhaust.

When I noticed the damage today I've made a compression test, which revealed nothing. All cylinders read 116 - 118 psi (cheap compression tester with a long tube)

After replacing the plug, the bike runs like a charm again.

So my question is: Is it possible to have a spark plug 'erode' this way from overheating? Or is this just a production failure or wear pattern on a spark plug that has run roughly 15,000 miles?

Cheers
Carsten

[lucky]

Yes that is from overheating.
The bike started slowing down that was a clue.

Could have been close to seizing.
It could have been getting low fuel flow since the plug color is fine.
Also the two plugs look different.

was the plug that had the short electrode on the carb farthest from the fuel?

100 miles one way....you must have been getting low on fuel.

[Fritz]

Also the two plugs look different.

You are right. My fault, looks I put the wrong plug into the pic (NGK DR7E). But #2's plug has the same color and condition of the electrodes.

was the plug that had the short electrode on the carb farthest from the fuel?

What does "farthest from the fuel" mean? Lenght of fuel lines? It's the plug of cylinder #1. Since carbs #1 and #2 are fed through a shorter line, than #3 and #4, I guess the answer is 'no'.
 

100 miles one way....you must have been getting low on fuel.

I forgot to mention, that I had a stop refueling after 50 miles.

So, you think it's overheating. That makes me wonder how this is possible. I definitely did not run low on fuel.
I cleaned the carbs two years ago and took great care that every single passage and jet was clear. The air filter is stock and the exhaust is known to not require rejetting (http://www.motad.co.uk/product.php?prod=CB550+F1-F3+Sohc+Four)

I guess, I should recheck float settings and tank venting.

Cheers
Carsten

[scottly]

The deposits indicate a hot plug. Are D7EA stock for a 550? Also, the sealing washer looks corroded, and doesn't look like it has been crushed at all. A plug that isn't properly tightened will run hotter, as it will not conduct heat into the head as well. The motor probably ran poorly due to the huge gap. 

[lucky]

Two years ago?  I would get some more shed time on those.
That one plug from your engine has the right color.
What about the other plugs?

[scottly]

Fritz, I forgot to mention there was a thread a while back about counterfeit NGK plugs..

[Fritz]

The deposits indicate a hot plug. Are D7EA stock for a 550? Also, the sealing washer looks corroded, and doesn't look like it has been crushed at all. A plug that isn't properly tightened will run hotter, as it will not conduct heat into the head as well. The motor probably ran poorly due to the huge gap.

Yes, I might have been too shy when I screwed the plugs in. I ran the bike during the winter, that's why the left plug's washer is heavily corroded.
D7EA is Honda's recommendation for the cb550, and I've got them from Yamiya in Japan.

Surprisingly, the bike idled fine with the "overgapped" plug.

[Fritz]

Two years ago?  I would get some more shed time on those.

Well I completely took the carbs apart so I could clean their bare bodies.

That one plug from your engine has the right color.
What about the other plugs?

The other plugs of #2 #3 and #4 all have the same color without any spots that might indicate overheating. That make me think that I do not have an overall lean condition.

What really puzzles me is the fact that the center electrode of #1 does not look like it melted but rather sheared off or "backed off" into the insulator. I think that the ground electrode would suffer from overheating as much as the center, but it looks fine.

Here's another closeup of the same plug.

[scottly]

What really puzzles me is the fact that the center electrode of #1 does not look like it melted but rather sheared off or "backed off" into the insulator. I think that the ground electrode would suffer from overheating as much as the center, but it looks fine.

Here's another closeup of the same plug.

I agree, if the plug had failed due to a lean condition, or detonation, the ground would show signs of melting. If the center electrode had melted, it would not have such a flat appearance. The cylinder pressures at idle are far less than at 100 MPH, so the stock system was adequate, even with the large gap.

[Fritz]

Thanks for all the feedback Scottly and Lucky!

Since the bike is running fine after replacing the damaged plug, I've decided that I will just leave it for now and make some plug readings during the next few weeks.

Maybe the center electrode just came lose and was pushed into the insulator.

I don't have much experience with motorcycles and I've put alot of money and time in rebuilding the engine with new oversize pistons, main bearings, valves, chains, etc just 8000 miles ago. So this failed spark plug really made me panic 

Cheers
Carsten

[scottly]

Once you see how the spark plug is constructed, you'll understand that if the internal resistor breaks down, it leaves space for the center electrode to be pushed upward, under repeated high compression events.  See plug construction pic below.
If the ceramic resistor crumbles or is eroded by a spark jumping an internal gap, the extra space allows the electrode to be driven upwards under combustion pressures, increasing electrode spark gap.  This is what your pictures show.


Cheers,

This was not a resistor plug that failed, Lloyd. Check NGK's website for pics of a too-hot plug, rather than Dan's.
http://ngksparkplugs.com/tech_support/spark_plugs/p3.asp?mode=nml

[TwoTired]

OK,  I'll remove my earlier post.

[scottly]

Lloyd, please repost the cutaway pic of the non-resistor plug; it did show the tip to be a nickel alloy bonded to a copper core? I suspect that tip somehow came loose on Fritz's plug, and that's why it looks like a clean break.

[Fritz]

Hello LLoyd,

OK,  I'll remove my earlier post.

Why that? The pictures are very helpful. Scott is right, the failed plug is a non-resisor type (D7EA), but maybe something else collapsed.

If I ever find the time, I'll try to cut it and see what happened inside. For now I am confident that the cylinder was not overheating and will keep an eye on the remaining plugs of the same batch in my bike.

Cheers
Carsten

[Irukandji]

Fritz,,, It could be a spark plug polarity issue. Stators or resistors or some such stuff. TwoTired and another member had a discussion about this melting the electrodes of the plug.

[TwoTired]

Fritz,,, It could be a spark plug polarity issue. Stators or resistors or some such stuff. TwoTired and another member had a discussion about this melting the electrodes of the plug.

I though about spark erosion.  But, that usually removes all sharp edges from the electrodes.  Due to two plugs firing at once, one has opposite polarity than the other, and the polarity determines which half of the electrodes deplete.  But, I don't see the effects of spark erosion on those plugs.

As requested, here is the spark plug internal structure picture.

[scottly]

Thanks. In the pic, it doesn't look like a straight interface between the core and the tip, but perhaps the illustration shouldn't be taken literally?

[TwoTired]

My 2¢...
I can't find a full length cutaway of a non-resistor NGK plug to study complete internal construction of the center electrode.
I can't think of a single man made product that is 100% identical in production, or one with zero defects for all production examples.

Were I a betting man, I'd bet that the plug failure depicted was that of electrode recession due to an internal defect.  Without dissecting the failed component, I'm not sure we will ever know.  Hard to know what is inside the house without opening the door or looking in a window.

I AM certain that the electrode wasn't melted by combustion temps., or eroded by spark energy (which would occur on two plugs rather than just one).

If the owner is insistent that the plug damage was from over-temp, I'd check the intake for air leaks, I suppose.

Best of luck!

[dave500]

maybe the porcelain insulator broke and slid downwards?

[trueblue]

I would have to agree with TT on this one, I would say that there was an internal failure and the centre electrode has pushed back into the porceline.  This could have changed the effective heat range of the spark plug and made it run hotter, giving you the deposits on the plug.

[dave500]

faulty manufacture?one in a million?

[trueblue]

faulty manufacture?one in a million?

It does happen, a couple of years ago, I saw 2 NGK plugs, in the same week, that dropped the centre electrode clean out of them.  The porceline was undamaged, just the electrode fell out, one made close friends with the top of the piston and the other made friends with the exhaust valve. 

[dave500]

bugger,i thought only champions were famous for that?remember them?they only come in lawn mowers now,not long ago i spotted a cheap chinese mini bike advert in a magazine,it was stating its good points like disc brake and what not,,and "fitted with ngk spark plug",,probably the best part of the bike?

[trueblue]

Probably wasn't even a real NGK plug in that chinese bike.  I once worked at a shop that only used champion plugs, I queried the boss about it one day, his reply was that they were the only plug that could guarantee return work .  It wasn't long after that I packed my toolbox up and left. 

[dave500]

ha!a lot of plug brands have fallen to the way side,whats the "KLG"equivalent of a D7EA?