Help with trouble shooting a failure

Hey Fellas,
If I were to post up some pics of a top end failure, would some of you guru's be willing to help me out with what caused the failure? I really don't want to repeat this problem if I can avoid it.
Thanks in advance, Griffbones

2007 KX100 engine, pipe, electrics and carb in a 2005 KX85 bike. 13/50 gearing with the short wheels of the KX85, needless to say it is geared pretty deep. We ride tight woods and my son is NOT very agressive thus the deep gears and a flywheel weight.
Last week I mixed up a fresh batch of 91 octane pump fuel mixed with Amsoil Dominator at 40:1. We ride for about an hour or so then head down a pretty long straight away. My son always rolls the throttle and then backs off and the rolls on then backs off on real long straights becuase I have told him not to leave it pinned on a real long (1/8 - 1/4 mile) straight. Well all at once the bike dies on him, but restarts but doesn't want to idle. We get home and the bike starts right up and idles fine. A compression test reveals 165-170 PSI! But the top end sounds noisey, so I pull it apart and find this:


Now this bike is jetted 100% stock, so I would have thought that it would have been a little rich. What few times he has ridden the bike the plug porcelin always looked tan with a bronze color to the base of the threads and on the electrodes.

The bike still seemed to run very strong and as stated had 165+ for compression but the top end had a rattle, so that is why I tore it down and found this problem.

Yesterday per the service manual, I smoothed up the cylinder with 440 grit wet dry sand paper, and then cleaned the cylinder with brake cleaner, then soapy water, then rinsed and then dried it off with compressed air. I then installed a new Wiseco piston and top bearing. Started her up and she sounded great. I let it cool down completely, then repeated this warm up/ cool down cycle, and then took it for an easy five minute ride. Then I let it cool down again and then started another easy ride but only made it about 10 minutes top, and then she seized up again. This was with no hard, only moderate throttle action. This time here is what I found, and the inside of the cylinder looked like it was full of gray lapping compound, unlike the first tear down which looked clean.


And here is what the plug looked like at each tear down, dark gray and almost powder coated, instead of the nice tan color that I had been accustomed to.


What the heck is causing this? And sorry for the long winded post, just trying to accurately describe the events. BTW this engine probably only had about 8 hours on it before the first failure. No jetting was changed so I don't think it should have been lean. And as I stated this was fresh gas with synthetic oil at 40:1.
Thanks a bunch guys, Griffbones

I did two warm up cycles and then a real easy five minute ride, then after another cool down I rode it for about ten minutes max at only partial throttle. And what caused the first failure? This thing had a tan plug when it was 50 degrees outside, but it failed on a 75 degree day, should have been even richer, no?

That imprint is where the piston appears to have drug on the intake port and rolled the metal up.

Could a CDI box go bad and throw the timing wacko and cause it to heat?

The first failure bothers me more than the second because the plug had been running so good and I just don't see the reason for that failure. I maybe should of had the cylinder re-plated before going another round, but I didn't think it looked that bad. After the second failure there was plenty of oil film in the cylinder but it was full of gray paste!

76GMC1500 said:

I had a CDI box go bad on my Yamaha which made the timing run way advanced at idle. But... I would expect some crown damage if timing were the cause of failure. 40:1 oil ratio is pretty lean for a 100 but I don't think that's the problem. Did you happen to measure the bore clearance before you put the new piston in?

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Most of the local guys are running 40:1 & 50:1 on the 100's and the 85's when using the Dominator oil. I am always confused by this because some people say to run the bike manufacturers specs, and others like Eric Gorr say to run what the oil manufacturer recommends for their product because different oils are different.

I didn't have the equipment (bore gauge) to measure the cylinder, but I did check the ring gap. What made this thing loose the first bone stock top end? What the heck changed from the previous times we had the bike out? This was an ultra low hours engine (brand new 2007 KX100 engine). I changed nothing as far as set-up and I always do maintenance ie. air filters, tranny oil, fresh premix, a compression test after every couple of rides, and many of my son's rides are short, like only an hour or two tops.

To me it looks like this thing got real lean real quick for an un-known to me reason, or it got real hot for some reason. The piston domes looked fine, in fact the original piston looked exactly like the photos Eric Gorr used to have up on his site, the ones where he says the transfer ports are nice and even and an example of good jetting!

The bike was not loosing any coolant either, so I don't think it got hot from that stand point.

The bike didn't sound lean either, except on the second top end, just a minute or so before it blew. I thought at the time it sounded higher pitched and not as deep sounding in the exhaust note. On the first top end I couldn't say, as I was following my son and couldn't hear his engine very well over my four stroke.

Man I just don't want to completely rebuild this thing top and bottom end just to have the same thing happen again, the wife is starting to get P-Oed at all the money I am spending on this bike. Especailly since our old 1983 CR60 has been just about bullet proof, and this new bike (bought last July as a left over 2005) lost the first KX85 engine to a crank failure almost imeadiately. That is how I happen to buy and put in this new 2007 100 engine.

The first two pics are of the original Kawi piston, it is badly scored on both the intake and exhaust. The third and fourth pics are of the Wiseco piston and it is scored really bad on the intake and only a line at the exhaust bridge. If this gives any help or clues to you guys.

I don't think the engine ever burned tranny oil, but I changed it after every single ride, no exceptions. As I said, the plug had always run a nice tan color, and I always checked it after every ride too, just becuase I wanted to avoid anything like this from happening.

Seems like I see a lot of kids running the crap out of these little bikes, their dad's do almost no maintenance and they run forever, I on the other hand am the most picky person you will ever find, yet I keep loosing these Kawi engines?

I had wondered if there was anyway at all that oil could go bad? I used a bottle of Amsoil that had been opened up last fall but then the cap tightly re-sealed, but it did sit in the garage all winter through zero degree temps. So I called up Amsoil, and of course they say as long as the cap was on the bottle to keep moisture out, there should be absolutely no problem with the oil.

Just to make sure I am going to throw away the bottle of oil and buy a new bottle today.

whenfoxforks-ruled said:

Do you know what bridge relief means?

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Not for sure. But doesn't it mean the chamfering around the bridge between the exhaust ports? Why would the factory original piston do this, especially after running for a few hours previous to the failure? Not trying to argue at all and I really really apreciate all of your help. I am just trying to wrap my head around this whole deal so that I can avoid it from happening again, I really can not afford ($$) to just guess.

2strokerfun said:

With the intake side on the new Wiseco torn up like that, it looks like lack of lubrication to me. Sure someone didn't put unmixed gas in it by mistake??

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I also thought it looked like a lack of lubrication, but I mixed the fuel myself, so I know for a fact it was 40:1, the same as I always mix it. But I agree it completely looks like no lube. When I was a kid I had a Suzuki TS185 that the injection pump quit on (due to a bad o-ring) while I was riding it, and this is exactly what that piston looked like. This is why I was questioning the possiblity of the oil going bad after setting in the garage all winter? I would not think so, and Amsoil didn't seem to think so either, but.......