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What causes this?

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ass003.jpg

The bike is a 2005 YZ125. The engine has been ported by Pro Circuit. I run a 50/50 mixture of 110 octane and 93 octane gas, and the plug looked good before this.

I was riding it tonight, and all was running awesome as usual for this bike, when it suddenly died blasting out of a corner. I kicked it about 8 times, and when it didn't fire I knew something was up so I pushed it to my truck to check the plug out, and it was obvious why it died.

I'm not expecting the exact answer as to what happened in my application, but what are the known causes for this? I know, I have to look at my own engine to see what the extent of the damage is.

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I'd go with lean condition detonation. Sometimes it'll break or crack the ceramic too.

It'll 'pock mark' the top of the piston too.

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Why does it look like your spark plug has coolant on it? That may have something to do with it??

That's not coolant.

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Was the bike pinging at all? If so, the running lean theory could be a possibility.

It's hard to say your bike is running lean from just looking at this pic since the pic is overexposed to the point that your fingers are very light colored, this same overexposure in the pic makes the plug ceramic look white rather than tan or brown.

If you're bike wasnt pinging at all, maybe it was just a defective plug. I take it the electrode strap blew out the exhaust? Or did it end up gouging up your piston/cylinder?

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Nope, no pinging. The bike ran perfect, and you're correct, the picture really doesn't look like what it does in person. The plug was a light brown color. I have yet to tear the top end apart, but I did drop the oil and it wasn't in the bottom.

We'll see what the deal is probably tonight.

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Since we're talking 2 stroke here...there really isnt any way for the broken electrode strap to have found it's way into your transmission oil since the cylinder/crank area is sealed off from the tranny. If you could find that piece in your pipe or silencer it would eleviate any concerns that it got sucked down into the crank area (which would of course be bad).

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looks like the piston hit the plug? I would try a double washer? my friend had a blaster with a 240 kit that would hit the piston and he double washered it and it never did it again. If you say it was running perfect, Id try it? :bonk:

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i had my wristpin bearing go out on my old rm 125...piston than hit the plug and head

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I would rule out piston hitting plug since the head is domed so it would be pretty damn hard for that to happen

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Stick a magnet on your crank & rotate it, if the electrode is in the crankcase it may come around. It more than likely went out the exhaust.

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This electrode has been melted off, detonation is what causes this, very lean and or to much ignition timing, the timing can be changed on pickup if it has slipped where it is bolted behind the flywheel.

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ass003.jpg

The bike is a 2005 YZ125. The engine has been ported by Pro Circuit. I run a 50/50 mixture of 110 octane and 93 octane gas, and the plug looked good before this.

I was riding it tonight, and all was running awesome as usual for this bike, when it suddenly died blasting out of a corner. I kicked it about 8 times, and when it didn't fire I knew something was up so I pushed it to my truck to check the plug out, and it was obvious why it died.

I'm not expecting the exact answer as to what happened in my application, but what are the known causes for this? I know, I have to look at my own engine to see what the extent of the damage is.

Holy Crap!

I have not seen that outside of a good running Top Fueler!

Couple things:

If it MELTED off, the plug will be Blue about 3-4 Threads down

If it's not Blue down on the Threads, it likely broke off.

Regardless, pull the Head and look at the Piston and see if there are "Dents" from the electrode touching.

If not, you got a bad Plug. It happens.

Make sure the Cylinder is not scrached and honestly you should try and find the broken peice if you can. It could be rattling in the Pipe if you are lucky.

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it was a common problem on Rm125s for years, the ground strap breaks off- maybe a little bit too much ignition advance and maybe some detonation- sometimes you cant hear the detonation. maybe you developed and air leak and it went lean?

whatever you find out, its always worth running the expensive plug from a late model RM125- they have a special plug that resists loosing the ground strap.

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