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Wash and plug look fine ( I think), piston broke (not melted) and the whole outer edge is rough. Just bought the bike and before I rode I inspected the best I could the piston and noticed the roughness on the outer edge but didn't think much of it. I don't think since I have ridden it (10 hrs) that it has had a detonation problem but maybe the last guy did.

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I'd say,

 

Signs of detonation (pitting around the edge of the piston, chunk broken):

either squish height needs correcting, octane level too low, jetting too lean.

 

Piston's crown too dark, no piston wash visible in line with intake ports:

jetting too lean for ambiant temps.

 

Add some clear pics of the sparkplug and cylinder head.

Edited by mlatour

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Luckily your cylinder head looks ok, not damaged.

 

If you look down into the spark plug under a light and using a magnifying glass,

see if there are any shiny deposits stuck on the center electrode's porcelain insulator.

If yes that is another sign of detonation.

 

As per this guide: http://www.braigasen.com/howtoread.htm

Edited by mlatour

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Plug porcelain looks great. I'm hoping it was the previous owner having the issues and weakened the piston. My cylinder escaped damage as well

.

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Take a pic of the inside (underside) of the piston...try to get the bottom side of the crown in focus. Look for a dark spot or a blue spot.

I'm guessing the detonation is due more to overheating than mixture...Maybe top end mixture is lean...but there is a little wash on the piston. It doesn't have that oiled golden color it should.

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No I wouldn't say the piston wash looks good:

 

https://www.google.ca/search?q=piston+wash+2+stroke&tbm=isch&tbo=u&source=univ&sa=X&ved=0CBsQsARqFQoTCMWtzIn2kMkCFYNGJgodH3gMww&biw=1437&bih=754#imgrc=_

 

Before you run this engine again you should measure the squish height and have it corrected if it's off.

If you don't already, you should always use premium high octane gasoline.

Once re-assembled and running,  you'll also have to check your jetting making sure it's not too lean.

 

As E36M3E mentions, post a pic of the bottom of the piston (underside of the crown).

Edited by mlatour

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No I wouldn't say the piston wash looks good:

 

https://www.google.ca/search?q=piston+wash+2+stroke&tbm=isch&tbo=u&source=univ&sa=X&ved=0CBsQsARqFQoTCMWtzIn2kMkCFYNGJgodH3gMww&biw=1437&bih=754#imgrc=_

 

Before you run this engine again you should measure the squish height and have it corrected if it's off.

If you don't already, you should always use premium high octane gasoline.

Once re-assembled and running,  you'll also have to check your jetting making sure it's not too lean.

 

As E36M3E mentions, post a pic of the bottom of the piston (underside of the crown).

stock jetting, always 93 octane  

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Deff has some burnt oil there under the crown and tons of carbon build up so probably indicates lean condition. ...Maybe intake leak, crank seal or reed cage seal.

Seal it up after your new piston is in. Build a test rig if you don't have one. Pressure check the cylinder and crank seals. If it holds pressure then you have a jetting problem.

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Check out the crank bearings while it's apart. should be no play in the crank at the mains, and not too much side to side on the rod. I agree, air leak somewhere?

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If you ride at 1/4 throttle or less most of the time could a air screw that's 2.5 out cause that? (50 pilot jet)

No up and down play at all

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Wash and plug look fine ( I think), piston broke (not melted) and the whole outer edge is rough. Just bought the bike and before I rode I inspected the best I could the piston and noticed the roughness on the outer edge but didn't think much of it. I don't think since I have ridden it (10 hrs) that it has had a detonation problem but maybe the last guy did.

You have zero wash, and the detonation is what ate the piston away, probably caused by overheating (which could have a number of causes) Black spot under the piston is also from running way hot

 

You should do a lot more research on two strokes before putting that back together or it will just blow up again.

 

A leakdown test before you pulled it apart would have been really helpfull to find possible air leaks, but most people are in a rush to to get it apart to think about this.

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Going to build a leak down tool but am skeptical that I will find a leak.

How does the plug look to you?

The outer rim of the piston is free of carbon, isn't this wash?

If it was that lean it would melt the piston and not break?

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The top of the piston is black cause you are lean and the carbon is burning on to it.

Wash is clean stripes or circular areas on the piston  at the edges in the area of the intake ports from fuel coming in washing  the piston crown clean. The outer edge of your piston is very rough and looks sandblasted. Compare that to the replacement piston and you will see a huge difference

 

Plug looks very dry and seems to have aluminum specs on the electrode just like the top of the piston from detonation.

 

An extreme lean condition can melt the piston, but that would be from a sudden massive airleak or fuel loss.

A slight airleak would make you run hot enough to detonate.

 

Check for air leaks, ignition timing to far advanced, crappy fuel, faulty CDI, compression ratio too high.

 

A leakdown tester is really easy to build. I do the test before assembling the clutch so I can see both crank seals and anything less than 0 drop over 6 minutes is not acceptable to me.

Making a tool to check ignition timing with a dial indicator and cut up spark plug is really simple too.

 

Check the crank carfully as detonation hammers the crank bearings, and crank bearing failure can be very ugly and expensive.

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If you ride at 1/4 throttle or less most of the time could a air screw that's 2.5 out cause that? (50 pilot jet)

No up and down play at all

 

If you use a YZ250 to ride around at 1/4 throttle or less most of the time you likely have the wrong type of bike for your usage.

 

Jetted right it can be ridden around at low rpms, but it still requires certain forward momentum for enough air to flow thru the radiators to cool the engine.

Edited by mlatour

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