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What would cause a spark plug electrode to desintigrate?

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1994 Honda CR125

We bought this 2 stroke off of Craigslist last year some time and I don't know how many hours the top end had on it. Anyway, sometime last July-August it had a major malfunction. The spark plug electrode seems to have desintigrated. Seems like excessive heat is the only thing that could cause this but how did it get THAT hot. The spark plug before this one was pretty black so I think it was jetted rich. But I've heard a lean jet makes for a hotter chamber. Anyone got any ideas how this could happen. I included some pics of the spark plug and the resulting damge to the piston and piston ring. The little pieces of metal that came off the plug got jammed up in between the the ring and ring grove and fused the ring into the groove. I've had the cylinder sent out to be replated and we got a new head and top end kit ready to go. I would like to know how this happened so that I could avoid this again.

Spark Plug.jpg

Piston 1.jpg

Piston 2.jpg

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Is the spark plug the proper one for the application ? (heat range)

Post some pics of the top of the piston.

 

Keep in mind if the engine experienced severe and prolonged pre-ignition/detonation,

the rod bearings likely got a beating as well.

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13 minutes ago, mlatour said:

Is the spark plug the proper one for the application ? (heat range)

Post some pics of the top of the piston.

 

Keep in mind if the engine experienced severe and prolonged pre-ignition/detonation,

the rod bearings likely got a beating as well.

Actually a BR9 is recomended and the desinitigrated plug is a BR8, which I know runs hotter. I think we put the BR8 in there because the other spark plug look rich fouled a little so we figured put a hotter plug in to burn off the fuel better.

I hope the bottom end is ok. I just planned on getting the new top end back in with it's new plated cylinder and hoping for the best. I can handle the top end. Cracking open the case and messing with the bottom end bearing makes a dark cloud of worry form in my head. That's an operation.

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Oh yeah, the top of the piston. The outlined area is just where the piston hammered the pieces of metal into the head. Other than that, the top of the piston looks good to me.

Piston 3.jpg

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2 minutes ago, Motox367 said:

You ride this bike wide open down the road?  Just curious 

It's my son's bike. I got a 2001 KX. Him and his buddy were riding the CR when it screwed up. I don't know the exact circumstances of what they were doing.

According to this link Krannie posted...

http://www.braigasen.com/howtoread.htm

..detonation is a timing problem I guess.

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Looks like old age of crank bearings causing lean conditions to me. And wide open throttle with no load. If spark plugs to hot others timming it burns hole threw piston.  I'd say you better do bottom end. Seen this many many times 

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7 minutes ago, Motox367 said:

Looks like old age of crank bearings causing lean conditions to me. And wide open throttle with no load. If spark plugs to hot others timming it burns hole threw piston.  I'd say you better do bottom end. Seen this many many times 

I think you're probably right. The bike is nearly 25 years old. Who know's if the bottom end has ever been changed. This sucks. I've looked at a youtube video of a bottom end being done. It don't look easy. CRAP!!!

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4 minutes ago, PigHead said:

I think you're probably right. The bike is nearly 25 years old. Who know's if the bottom end has ever been changed. This sucks. I've looked at a youtube video of a bottom end being done. It don't look easy. CRAP!!!

If you're close to nylook me up lol bring parts be done that night 

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Just my two cents , the spark plug did not cause this damage, you say you dont know last time the top end was done, looks to me the damage to the piston was cause from overdue top end rebuild resulting in major piston slap =bang, spark plug was just in the line of fire,

 

for reference i run br8eg in both our CR's 250 and 80.

top ends last 20-30 hours on small bores .

 

 

 

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14 hours ago, Motox367 said:

If you're close to nylook me up lol bring parts be done that night 

Thanks for the offer. I'm gonna have to learn to do it myself. I can do a lot with my own vehicles. I got a lot of tools and what I don't have when I need something, I buy it. For the price of a tool or two I can save myself hundreds of dollars in labor charges rather than having some mechanic in some shop do it. And then I have the tools I bought for that fix for later fixes. Win win.

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16 hours ago, CRJohnny said:

Just my two cents , the spark plug did not cause this damage, you say you dont know last time the top end was done, looks to me the damage to the piston was cause from overdue top end rebuild resulting in major piston slap =bang, spark plug was just in the line of fire,

 

for reference i run br8eg in both our CR's 250 and 80.

top ends last 20-30 hours on small bores .

 

 

 

Thanks for the input. You know I was thinking there for a minute did the spark disintigrate on it's own due to overheating and cause the damage or was it reverse? Did something cause the damage to the plug? Man I got this top end rebuild ready to be put back together. New plated cylinder dipped in nickle ($280), new Wiseco top end kit ($130), and a brand new hour meter to stay on top of it ($5.00!). AND we got a brand new fatty gold series pipe to go back on too ($200-birthday present to son). I want this thing back together and running.  This thing has been sitting there for months. I think I can handle a bottom end rebuild and it probably does need a rebuild. I'm gonna ask my son what he wants to do. Throw the top end back together and go with it or go through with a bottom rebuild.

Edited by PigHead

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"Some engines, such as liquid cooled 2-stroke engines found in snowmobiles, watercraft and motorcycles, have a very common detonation failure mode. What typically happens is that when detonation occurs the piston expands excessively, scurfs in the bore along those four spots and wipes material into the ring grooves. The rings seize so that they can't conform to the cylinder walls. Engine compression is lost and the engine either stops running, or you start getting blow-by past the rings. That torches out an area. Then the engine quits." Quoted from this article, looks like exactly what happened to your rings and piston edge.

http://www.contactmagazine.com/Issue54/EngineBasics.html

 

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16 minutes ago, kdxyardsale said:

"Some engines, such as liquid cooled 2-stroke engines found in snowmobiles, watercraft and motorcycles, have a very common detonation failure mode. What typically happens is that when detonation occurs the piston expands excessively, scurfs in the bore along those four spots and wipes material into the ring grooves. The rings seize so that they can't conform to the cylinder walls. Engine compression is lost and the engine either stops running, or you start getting blow-by past the rings. That torches out an area. Then the engine quits." Quoted from this article, looks like exactly what happened to your rings and piston edge.

http://www.contactmagazine.com/Issue54/EngineBasics.html

 

That sounds like what my son described as happening that day.

Reading further in the article, it also says that detonation, which occurs when unspent fuel/air mixture is left in the chamber (after the initial spark plug iginition of the fuel) spontaneously ignites from high temp and/or compression which then creates a shock wave which can break the spark plug electrode. So then how to prevent detonation? Proper timing, proper octane fuel (which we use).

I just bought a bottom end kit so I'm goin ahead with that on top of the top end kit. Gonna have new crank, new crank bearings, new piston and ring. Then I'm gonna get a timing light and check the timing. I'm of the understanding the timing can't really be adjusted though. But I wanna at least check it to see where it is.

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I wouldn't worry about the timing but absolutely check the jetting for too lean. Lube the cylinder wall with straight 2 stoke oil on assembly wiseco pistons have a dual ring with opposed gaps, cycle the cylinder without the top on and make sure it is tight and wipes the lube you out on. I would say either too lean caused this or worn rings. Start with 30:1 on break in then I always ran 50:1 amsoil for power valves.

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11 hours ago, kdxyardsale said:

I wouldn't worry about the timing but absolutely check the jetting for too lean. Lube the cylinder wall with straight 2 stoke oil on assembly wiseco pistons have a dual ring with opposed gaps, cycle the cylinder without the top on and make sure it is tight and wipes the lube you out on. I would say either too lean caused this or worn rings. Start with 30:1 on break in then I always ran 50:1 amsoil for power valves.

50:1? Really. I've never raced or anything but we like messing around with dirt bikes. 50:1 is almost double the recomended ratio. You sure that thing is getting enough oil at that? Would that create a better situation for complete burn of the fuel by the plug spark so as to prevent detonation, which is a secondary explosion of unspent fuel after the spark?

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I got a good question for anyone too. If I don't get an answer here on this thread then I'm gonna have to make a new thread for it in hopes of getting some input.

So I sent my cylinder out to US Chrome in Wisconsin to have the bore replated and I just got the cylinder back (normaly a $230 job but mine was $280 because it had previously had a steel sleeve put in and that caused them to have to do something extra to replate). So I got my cylinder back and everything inside the bore looks nice and shiney and smooth. Like a stainless steel table buffed to a high shine. except for one section near the exhaust ports. Is this ok or is this something I should contact US Chrome about? Check it out. It's like a lightly scuffed area. It's pretty smooth to the touch, but the absolute shiney area in the rest of the bore is even smoother. Would this cause a problem? The top of the piston goes down below the exhaust ports correct? So the rings are deffinitely gonna rub past this area.

 

Cylinder Replated scuffs.jpg

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7 hours ago, PigHead said:

50:1? Really. I've never raced or anything but we like messing around with dirt bikes. 50:1 is almost double the recomended ratio. You sure that thing is getting enough oil at that? Would that create a better situation for complete burn of the fuel by the plug spark so as to prevent detonation, which is a secondary explosion of unspent fuel after the spark?

Yes 50:1, I raced enduros for years. Most guys I know running AmsOil go 60:1. It is all about the quality of your oil, dont do that with auto store 2 stroke oil. I have used Maxima Super Lube at the same ratio. 2 strokes with power valves mixed at 30:1 require cleaning very frequently. Best indicator is the "spooge" at the end of the silencer. Below 100cc always use 30:1 they have a lot higher redline. Reducing the ratio can effect your jetting response as well, so you may have to tinker with both. It has no effect on detonation or fuel burn, it is about not having overlubrication, pull your exhaust and look inside the pipe where it connects to the bike. At 30:1 most will have a 1/8" build up of goop. A properly ringed motor with the right plug heat range and jetting does not have detonation. Timing can effect that too but I have never had an issue with that, I am 51 and have rode bikes since 5. 

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7 hours ago, PigHead said:

I got a good question for anyone too. If I don't get an answer here on this thread then I'm gonna have to make a new thread for it in hopes of getting some input.

So I sent my cylinder out to US Chrome in Wisconsin to have the bore replated and I just got the cylinder back (normaly a $230 job but mine was $280 because it had previously had a steel sleeve put in and that caused them to have to do something extra to replate). So I got my cylinder back and everything inside the bore looks nice and shiney and smooth. Like a stainless steel table buffed to a high shine. except for one section near the exhaust ports. Is this ok or is this something I should contact US Chrome about? Check it out. It's like a lightly scuffed area. It's pretty smooth to the touch, but the absolute shiney area in the rest of the bore is even smoother. Would this cause a problem? The top of the piston goes down below the exhaust ports correct? So the rings are deffinitely gonna rub past this area.

Yes the piston goes above and below every stroke. I have had a cylinder sleeved before to the stock size and mine did not have that. I dont think it is an issue as anytime the piston and rings are passing this area there is an air leak anyway in to the exhaust ports.  I would call them anyway to see if this is a normal look for their service, but the main thing is it does not stick out in the chamber. It may be they overground this area and if they polished it to look mirrored they would be making it even deeper away from the piston and rings.

 

 

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