YZ 250 detonation?

acerbis
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acerbis
Edited Date/Time 7/30/2016 2:09pm








So I bought a 2007 YZ 250 and after one lap at Glen Helen this happened. The bike never really had any power at the track and I had to clutch it big time going up the hills, so I think this was an existing problem. When I test rode it, it had great compression, stated 2nd kick and it pulled the front tire off the ground easily. Any ideas on what I can do to prevent this in the future?
I think i'm going to replace the crank seals just to be safe and run a 50/50 mix of race gas, but could this be caused by anything else?

Thanks for the input.

92 octane, stock needle on the second clip. Pro-circuit pipe and silencer.
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Paw Paw 271
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Paw Paw 271 7/28/2016 2:45pm
7/28/2016 2:45pm
The look of the piston indicates that the crank came apart, not detonation, unless it was the spark plug tip that came off.

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acerbis
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acerbis 7/28/2016 3:14pm
7/28/2016 3:14pm
the crank seems fine no up/down movement.
Jakes Dad
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Jakes Dad 7/28/2016 4:32pm
7/28/2016 4:32pm
That is not detonation, that is debris.
CamP
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CamP 7/28/2016 5:11pm
7/28/2016 5:11pm
Needs a crank

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pete24
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pete24 7/28/2016 6:30pm
7/28/2016 6:30pm
follow the grove in the piston it leads rite to where the clip used to be, crank or no crank your taking the bottom end apart and cleaning it out
ridinbeast932 7/29/2016 8:08am
7/29/2016 8:08am
Looks like most of that damage was from debris but if you look closely around the edge of the head it looks like there may be some pitting from detonation? Either way, 92 octane is too low for that bike. In 2011 they lowered the compression to help with detonation but I still run a gallon of race fuel to 3 gallons of 93 in my 2014. First YZ out of like 5 (pre 2011) that I haven't had detonation issues with. Knock on wood. Definitely go for a 25/75 or 50/50 combo of race/pump once you get it back together.
rippinruts 7/29/2016 9:05am
7/29/2016 9:05am
The gas you run shouldnt be the problem. Im from Norway and they dont sell race gas at the gas stations, I have to special order them i think. So my whole life I have been running 95 octane and 98 octane regular gas. On my YZ125 I use 1 Liter oil with 30 Liters of gas (30:1 or 33,3%). Which is what it says in the owners manual. I dont have any mods to the bike though, which in your case is the pipe and silencer. Hope you figure it out Smile
Dirtdemon 7/29/2016 9:42am
7/29/2016 9:42am


Some metal part (most likely from the crank bearing) was traveling upwards, if you had stopped you would have saved the head.

acerbis
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acerbis 7/30/2016 6:45am
7/30/2016 6:45am Edited Date/Time 7/30/2016 7:29am
My initial thoughts were debris. The crank turns over super smooth but i don't want to have the same problem once I get a new top end so I'll take it apart and put in a new crank. Thanks for all of the replys. Hopefully I find the culprit.
Moto520
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Moto520 7/30/2016 7:58am
7/30/2016 7:58am
They detonate with 93 octane gas. Go 50/50 with the race gas. I don't know if that's what caused your problem but go 50/50 just to be safe.
dedi684
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dedi684 7/30/2016 1:32pm
7/30/2016 1:32pm
Only yz 250 i ever owned that detonated had a pro circuit pipe. Ive run straight pump gas in all of them. 02 06 two 09's and now a 16.
project racer 7/30/2016 9:08pm
7/30/2016 9:08pm Edited Date/Time 7/30/2016 10:02pm
That trench in the piston ,along with the two marks in the piston pin / clip area, looks about the size of a roller from a rod bearing that got spit up. Looking at the cylinder, the top edge of the transfer port, along with the top edge of the piston, above the rings, tell the story.

Something came through that port ,more than likely as the piston was on it's up stroke, it contacted the errant metal. The piston appears to have the heaviest impact marks above the ring, as far as depth of the groove. The damage on the cylinder above the transfer port also indicates it caught it on the upstroke.

Once it lunched the debris on the upstroke, it took it for a round trip ride back down to the basement.

The chip in the plating just above the port ,and no damage to the cylinder above the damaged port opening port is almost a certain indicator something came up through that transfer. Also note the gouge in the piston cut away below the pin bore.

Most likely, it banged the metal on upstroke, then the cutaway of the piston grabbed it on the way back down.

Only thing in the crank case / transfer area to be ingested through the transfer ports are the lower rod bearings, or thrust washers , if applicable.

Don't see much , if any detonation indicated on the piston. What got the head was tramp metal from the failure being digested as the engine turned it's last few revolutions as it came to a halt.



acerbis
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acerbis 8/2/2016 9:30am
8/2/2016 9:30am
project racer wrote:
That trench in the piston ,along with the two marks in the piston pin / clip area, looks about the size of a roller from a...
That trench in the piston ,along with the two marks in the piston pin / clip area, looks about the size of a roller from a rod bearing that got spit up. Looking at the cylinder, the top edge of the transfer port, along with the top edge of the piston, above the rings, tell the story.

Something came through that port ,more than likely as the piston was on it's up stroke, it contacted the errant metal. The piston appears to have the heaviest impact marks above the ring, as far as depth of the groove. The damage on the cylinder above the transfer port also indicates it caught it on the upstroke.

Once it lunched the debris on the upstroke, it took it for a round trip ride back down to the basement.

The chip in the plating just above the port ,and no damage to the cylinder above the damaged port opening port is almost a certain indicator something came up through that transfer. Also note the gouge in the piston cut away below the pin bore.

Most likely, it banged the metal on upstroke, then the cutaway of the piston grabbed it on the way back down.

Only thing in the crank case / transfer area to be ingested through the transfer ports are the lower rod bearings, or thrust washers , if applicable.

Don't see much , if any detonation indicated on the piston. What got the head was tramp metal from the failure being digested as the engine turned it's last few revolutions as it came to a halt.



Nice analysis.
I took the crank out and couldn't find anything...which is a bummer since I want to avoid this in the future.
rongi#401
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rongi#401 8/2/2016 10:29pm
8/2/2016 10:29pm
thats just how they run.
xnationalwrench 8/3/2016 5:43am
8/3/2016 5:43am Edited Date/Time 8/3/2016 5:45am
If the crank looks good in a truing stand look at the power valve to make sure it didn't come apart and the and the reed valve. Even check the air filter box because something got into the compression chamber to do that damage. Check the main bearings to make sure nothing is bad there also.
pete24
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pete24 8/3/2016 8:50am
8/3/2016 8:50am
if the crank is good and you cant see anything else then im sticking to my piston clip explanation
slipdog
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slipdog 8/3/2016 9:46am
8/3/2016 9:46am
Seen similar damage from the cage of the big end cracking apart and a small piece getting out and up the transfer. Inspecting the crank there was no play and seemed good until I noticed two of the needle bearings touching each other looking through the oiling slot on the rod.

Also similar from one of the rivets of the main bearing cage popping out from an almost new main bearing.

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