What caused this

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9/16/2017 6:40 PM

Kx 144 piston after 4 hrs. The jagged thing u see on the top of the piston is the spark plug tip.Photo

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9/16/2017 7:57 PM

Congrats, looks like you've melted a piston. Is the underside of it black? Did you change your jetting after going from a 125 to 144? What octane fuel and oil ratio you run?

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9/16/2017 8:02 PM

Detonation I would say. Get a better picture of the top f the piston.

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Tomac and/or Anderson for 2020.....

9/16/2017 8:02 PM

Flux capacitor let go

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9/16/2017 8:10 PM

Could be a number of things. air leak? water pump failure? shit jettting?

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2019 TX300
'10 200 XC-w

9/17/2017 9:22 AM

Sprocket bolts

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RTFM & AYFV

9/17/2017 10:39 AM

Big time detonating from poor fuel, jetting, aggressive ignition and head setup.

People moan about the cost of the oem spark plug on the rm125, but it's well worth using it on any 125 imo since it resist the ground strap coming off .

That said, that bike had major problems before the plug let go.

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9/17/2017 4:11 PM
Edited Date/Time: 9/17/2017 4:13 PM

ferg722 wrote:

Congrats, looks like you've melted a piston. Is the underside of it black? Did you change your jetting after going from a 125 to 144? What octane fuel and oil ratio you run?

It's set up for pump fuel and I run 32:1 with 92 octane. The underside isnt black tho

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9/17/2017 4:12 PM

Knoby wrote:

Sprocket bolts

Thanks. But that was the first thing I checked after it blew up. I already concluded that it wasnt the sprocket bolts.

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9/17/2017 4:19 PM

Photo

Photo

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9/17/2017 5:25 PM

Detonation

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9/17/2017 7:20 PM

Bruce372 wrote:

Detonation

X2..... Most likely cause for this is that the head to piston squish was not reset for the larger piston. With out the squish being reset it will cause a lack of lubrication to the edges of the piston and cause sever over heating. The over heating will also cause the plug tip to break off and then stamp the top of the piston. It also causes the pitting on the outer edge of the piston. The piston edges get so hot that it deforms and sticks the piston rings and you loose compression.
The head must be recut to match the oversized piston and create the correct squish clearance. You will also need to be sure to use the correct spark plug for that bike and not a knock off or cross referenced spark plug to be as sure as you can that the plug tip will not break again.
You will also need to do a leak test and a vacuum test to be sure you are not creating a lean condition.
Your jetting will also need to be checked and corrected as it looks to be running lean.
These 144 kits are not just a drop in and go item. They do work great when set up correctly.

Paw Paw

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9/18/2017 12:21 AM

If the head looks similar my guess would be either you sucked some debris in through the intake or your rod bearing is starting to go. Inspect the rod big end very closely, particularly the cage.

Otherwise, probably detonation.

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Retired Mechanical Engineer, published technical writer, mscperformance.com, Bisimotoengineering.com,

9/19/2017 3:13 AM

You can't go wrong with NGK. I think they're standard in Yamahas. They're also like $12.95 AUD here. CR8A for YZ125

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