YZ250 Blown Top End w/ less than 2 hours

Grizz
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Grizz
Edited Date/Time 3/2/2021 1:08am
2019, less than 2 hours on the entire bike. Was running 30:1 like the manual calls for. Had to have been bad pump gas, right?




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FGR01
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FGR01 2/26/2021 4:14pm
2/26/2021 4:14pm
Looks like it snagged a ring on the power valve. The power valve must have been extended excessively into the bore. Might be something wrong with the power valve mechanism up top that allowed that to happen.
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Paw Paw 271
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Paw Paw 271 2/26/2021 5:41pm
2/26/2021 5:41pm
FGR01 wrote:
Looks like it snagged a ring on the power valve. The power valve must have been extended excessively into the bore. Might be something wrong with...
Looks like it snagged a ring on the power valve. The power valve must have been extended excessively into the bore. Might be something wrong with the power valve mechanism up top that allowed that to happen.
X2 but I also see what looks like it has a squish issue. If so the excess heat created cause the ring to become loose in a distorted piston ring land and thus distort and cause the failure.

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Grizz
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Grizz 2/27/2021 6:32am
2/27/2021 6:32am
How can I avoid that happening again? I bought new OEM of everything that was damaged but didn’t buy any power valve stuff.
Mr. Robinson 2/27/2021 7:17am
2/27/2021 7:17am Edited Date/Time 2/27/2021 9:41am
The etching on the piston and head are signs of detonation. Once that starts temps will escalate quickly and it will begin to burn off the exhaust side of the piston crown. Molten aluminum starts to build up in the exhaust port, the ring becomes over heated, soften, looses support from the burnt off crown and hangs up on the exhaust port or the build up of aluminum in the port from the piston.

Detonation comes down to the lack of necessary fuel or octane for the combustion conditions. IE, the amount for compression, spark advance, squish clearance, lean jetting and so on. Low octane pump gas with a lot of ethanol that has set more than a couple of weeks is a very good catalyst for something like this.

Since this appears to be a totally stock, new engine, I would chalk this up to bad fuel or possibly clogged jets.
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Grizz
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Grizz 2/27/2021 8:12am
2/27/2021 8:12am
Oh and I forgot to mention that the plug didn’t look lean so it had to have been bad gas. Looks like I’ll need to locate some higher octane fuel near me.
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Mr. Robinson 2/27/2021 9:55am
2/27/2021 9:55am
The plug won't be a good indicator in a failure like this. It was probably dark like the top of the piston and that's basically aluminum and oil mixing together. If the engine is able to stay spinning very long after the compression drops way off, things will start to cool down about as fast as they heated up and your plug won't tell the tale of what really happened.
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katoom350
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katoom350 2/27/2021 4:41pm
2/27/2021 4:41pm
This has happened on more than one occasion. Some will detonate on pump fuel. You might have to run race gas
Bruce372
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Bruce372 2/27/2021 4:43pm
2/27/2021 4:43pm
Modern 2 strokes are on the edge of detonation, and today's pump fuel is so bad it's very easy to tip over the edge.

Imo 50/50 leaded race fuel
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Grizz
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Grizz 2/27/2021 5:29pm
2/27/2021 5:29pm
Bruce372 wrote:
Modern 2 strokes are on the edge of detonation, and today's pump fuel is so bad it's very easy to tip over the edge. Imo 50/50...
Modern 2 strokes are on the edge of detonation, and today's pump fuel is so bad it's very easy to tip over the edge.

Imo 50/50 leaded race fuel
Thanks for the help guys. Pretty frustrating with a new bike, this is why I like my four strokes better haha. Definitely going to get some race gas as I dumped a bunch of money on this two stroke as I’m planning on keeping it for a while.
DB505
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DB505 2/28/2021 9:36am
2/28/2021 9:36am
Bruce372 wrote:
Modern 2 strokes are on the edge of detonation, and today's pump fuel is so bad it's very easy to tip over the edge. Imo 50/50...
Modern 2 strokes are on the edge of detonation, and today's pump fuel is so bad it's very easy to tip over the edge.

Imo 50/50 leaded race fuel
Yes and make sure it’s fresh gas! Don’t let it set in the can for a week or two!
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DB505
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DB505 2/28/2021 9:39am
2/28/2021 9:39am
Bad gas will cause problems four stroke too!
A two stroke is soooo much easier to work on ! You just had some bad luck with this!
captmoto
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captmoto 2/28/2021 12:22pm
2/28/2021 12:22pm
MXA has mention a few times that they cut the pump gas with race gas on stock YZ250s. I can't find it but I think they went from 25% races gas to a 50-50 mix.
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timobijnen84 3/1/2021 6:49am
3/1/2021 6:49am
If you dont want to run race gas you can retard the timing 2° and you can get away with using high octane gas.
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CKNY
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CKNY 3/1/2021 4:32pm
3/1/2021 4:32pm
timobijnen84 wrote:
If you dont want to run race gas you can retard the timing 2° and you can get away with using high octane gas.
I used to do 2 deg like you say to run on 91 pump without detonating. I’d run Torco to help on hot days or sand.
yamaha2000
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yamaha2000 3/2/2021 1:08am
3/2/2021 1:08am
Looks like detonation caused by shitty fuel. Go to airports and start mixing some 100LL (av gas) 50/50. It’s like 4$ a gallon and I have used it in all my mod motors with 0 problems for years now
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