YZ 250 Breakdown Please Help

[trial_07]

Hi,
I'm posting for a friend. Here's what happened to his 2007 YZ 250 barely a few minutes after starting the bike today:

The bike always had a detonation problem after installing a complete FMF exhaust and re-jetting. This summer, the tip of the spark plug had fallen loose in the cylinder while running, damaging the piston and cylinder cap. Fortunately, the cylinder showed no sign of harm so he did not replace this part.

Specs:
Gas: 2/3 91 octane pump gas + 1/3 VP U2 mixed 32:1 with ELF oil
Jetting: 198 main, 45 pilot
Plug: BR8EG
The piston and rings on this picture had approximately 23 hours.

Any thoughts? Thanks!

 

[skinindustries]

For an 07 that has some rusty ass cylinder bolts. And as for your question I have never seen that happen before maybe I'm just lucky? How old is the piston itself? That could be the problem or it was jsut mechanical faluire

 

[whenfoxforks-ruled]

Need a pic of the top and under side of the piston. Them studs look like that from the meltdown. Very nice job by the way!

 

[trial_07]

I will post pics of the top and bottom of the piston tomorrow. Yes, the studs are rusted, you'd think the meltdown has something to do with that? Nice job for what?

skinindustries - The piston has about 23 hours, I should have written it on the pictures! The bike is a 2007, and I don't know a lot of people who disassemble the head to clean the studs. Steel rusts.

 

[whenfoxforks-ruled]

Your friend did a very nice job of blowing up that engine. It has been a long time since I have seen the cylinder skirt cracked off a 2 stroke. See if Eric has a big bore cylinder laying around. A safe bet would be changing the bottom end also. He has that too.

 

[trial_07]

I think he would be interested with a big bore. I'll try to get in contact with Eric. Does he come on DRN often? Thanks for the help! Any thoughts on what could have caused this?

 

[whenfoxforks-ruled]

He took an unknown unleaded, 91 octane, oxygenated pump fuel, and mixed it with 101 octane leaded, oxygenated race fuel. The concoction did not work?

 

[Deadohiosky39]

Eric comes on here sometimes but he is also a busy guy. I'd try this:
http://ericgorr.com/
Email: [email protected]
Telephone: 1-630-825-5645

I've never used him but i've heard nothing but good things.

 

[trial_07]

He took an unknown unleaded, 91 octane, oxygenated pump fuel, and mixed it with 101 octane leaded, oxygenated race fuel. The concoction did not work?

That would be a question best answered by Rich. My friend has been running for a while with this concoction.

Another thing I forgot to mention: The bike always had high compression. I have no numbers though.

 

[trial_07]

Here are more images:

Thanks for the input!

 

[FNG]

HOLY DETONATION BATMAN!!!!

Nice sandblasting around the edge of the piston.

 

[_JOE_]

Cha ching! Looks to me like one of the skirts broke off and got wedged on the down stroke which broke the cylinder. Then all hell breaks loose and things get beat to death. Got the cases opened up yet? Keep your fingers crossed they're not junk. What brand of piston was that?

 

[_JOE_]

And he's wasting money mixing those fuels. Most 2-strokes will run fine on 93 octane pump gas. The bike would most likely run better on that than the fuel stew he's dumping in the tank now.

 

[whenfoxforks-ruled]

No detonation. If the rod bearings are still in one piece, it was the piston skirt that let go. Loose bore, skirt past spec, foreign material in through the intake, or just left it pinned till it let loose? If it truly is high compression, it will greatly shorten the service intervals from the manual. Was it over 13.5:1 compression? I can assure you there is no way of knowing what your friend is putting in his gas tank, octane rating or oxygenate level. Short of sending some to a lab. He would be better off finding a gas station with alleged race fuel, out of the ground, sunoco 112 works out to about the same price as the concoction per gallon. Shell around here has rocket brand and it is even cheaper. But, that is around here.

 

[trial_07]

So you guys think the piston broke first?

 

[_JOE_]

That's my hunch.

 

[yathump]

My question is is that a OEM piston that you put in it? and did you match the piston to the cylinder stamp(A, B, C, D), if had a A cylinder and put a D piston in when the piston expands it could have stuck the piston causeing the failure

 

[_JOE_]

My question is is that a OEM piston that you put in it? and did you match the piston to the cylinder stamp(A, B, C, D), if had a A cylinder and put a D piston in when the piston expands it could have stuck the piston causeing the failure

Uhhh, that would most likely cause a seizure. More likely an A piston in a D cylinder, or just plain worn top end. If the piston to cylinder clearance is too large the piston can rock causing the skirt to slap on the cylinder. I would suspect this may be some of the "detonation" mentioned in the first post.

 

[whenfoxforks-ruled]

Seriously question, who modded the motor and declared it high compression, and is the rod still good? The damage shown, although blurry, around the squishband, really looks like the rod went. Any hot spots on the rod? The 1st pick sure looks like 2 globs of melted aluminum laying over the right transfer, the motor ran a while after it broke and is 2 pieces of well hammered skirt. Most of the engines with holes and skirts gone, are from the rod bearing(s) failed. That steel cylinder skirt did not jump off from no stink'in aluminum. Steel on steel. There is an outside chance that the piston skirt locked the crank journals, but there is the squish damage?

 

[trial_07]

My question is is that a OEM piston that you put in it? and did you match the piston to the cylinder stamp(A, B, C, D), if had a A cylinder and put a D piston in when the piston expands it could have stuck the piston causeing the failure

Now that you bring that up: The cylinder has been machined by a well known shop here in Quebec. However, my friend did not send the piston with the cylinder, but simply said it was size A. He will mesure the cylinder tonight in order to find out if it was machined correctly. As far as the high compression, I heard this was an on-going issue with the YZ250's for a few years now?

 

[_JOE_]

I'm putting my chips on the cylinder being a bit over-sized. Have you guys looked at the clearance between the piston and cylinder?

 

[yathump]

Uhhh, that would most likely cause a seizure. More likely an A piston in a D cylinder, or just plain worn top end. If the piston to cylinder clearance is too large the piston can rock causing the skirt to slap on the cylinder. I would suspect this may be some of the "detonation" mentioned in the first post.

I realize that i just neglected to write it

Too much blood in my coffee stream :worship:

 

[trial_07]

Seriously question, who modded the motor and declared it high compression, and is the rod still good? The damage shown, although blurry, around the squishband, really looks like the rod went. Any hot spots on the rod? The 1st pick sure looks like 2 globs of melted aluminum laying over the right transfer, the motor ran a while after it broke and is 2 pieces of well hammered skirt. Most of the engines with holes and skirts gone, are from the rod bearing(s) failed. That steel cylinder skirt did not jump off from no stink'in aluminum. Steel on steel. There is an outside chance that the piston skirt locked the crank journals, but there is the squish damage?

The rod is fine, no sign of damage or whatsoever: it looks like new and so is the bearing. Cylinder size A tolerances are 66.400 mm - 66.402 mm. My friend measured 66.40 mm, he does not have the tools required to measure a thousandth of a mm. I'll be able to measure it next week at school. The piston is size A.

 

[whenfoxforks-ruled]

I heard a story, once, of a guy who likes his bikes to detonate, a little. Seemingly he thought he could jet around detonaton? My book says: You can have a lean condition, or detonation. Lean is lean, and detonation is advanced firing do to high compression and mis-placed pressure waves. Detonation usually takes large chunks of the piston off the exhaust side. Lean will overheat and cause seizure. But the blurry top of the piston shot still looks like the squish hammered a lot of small debris before it ended. Where did all the small debris come from? Rod bearing or debris through the intake? The metric math is fine enough, I have a translator! Bore is seemingly good. If a bridge was tight, it sure did not show on the piston, or what was left. How do you measure excessive skirt wear now? How about the crank main bearings? Still tight? And these were checked after a thorough cleaning? Dirty bearings can feel tight.

 

[trial_07]

Yes, rod and bearings are ok. You think the damage on the top of the piston were from debris that entered the cylinder before the breakdown? And could be the actual cause of the breakdown?

How do we solve a detonation problem? Does it depend only on gas and level of octane?

When it broke, it did not make any noise, more like just fouling a plug. My friend realized it was much more, however, once he tried kicking it back to life.