Why did my YZ250 blow up

sesker15
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sesker15
Edited Date/Time 3/25/2014 1:57am
I ask thumpertalk. Thought I would ask my vital friends. 2006 yz250 32:1. About 20 hours on oem piston and rings.


What I think maybe what happen is. Air leak( makes it run lean), bad gas, detonation
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xnationalwrench 3/3/2014 2:13pm
3/3/2014 2:13pm Edited Date/Time 3/3/2014 2:15pm
Looks more like the piston collapsed, when you put the piston in did you check the clearance? Air leaks can cause seizures but not usually do that, unless you rode several times and seized it each time.
Jakes Dad
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Jakes Dad 3/3/2014 2:43pm
3/3/2014 2:43pm
What he said, too much piston to wall clearance. They usually shatter at the skirt from that.
pete24
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pete24 3/3/2014 2:46pm
3/3/2014 2:46pm
the erosion around the outside of the piston, from either detonation or bad gas or most likely both either way that thing has been pinging like a son of a bitch for quite some time and your friend is deaf and dum, shops wont tell u and people can claim whatever they want but a yz 250 any year needs a minimum of haf race gas in it , ur going to need to do the bottom end on that thing check the timming with a dial indicator when you put it back together, along with the squish
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Paw Paw 271 3/4/2014 3:10am
3/4/2014 3:10am
The spark plug photo is very blurred, but looks very white and the piston shows signs of running lean and low octane fuel.

The Shop

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sesker15
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sesker15 3/4/2014 7:56am
3/4/2014 7:56am
floridaflash wrote:
what fuel were you running and what jetting
Fuel pump 93. Stock 2006 jetting. Except one up on the main. Fmf pipe and siliencer. The fuel I had I think went bad.
floridaflash
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floridaflash 3/4/2014 8:37am
3/4/2014 8:37am
I would not run pump gas, if you are a fast, hard rider run race gas or at least a mix of pump and race, I prefer race alone. It looks like it detonated, but sometimes it is hard to tell what happened first. I would split the cases to make sure all the pieces are out. make sure you put new crank and main bearings in it also while it is apart.
sesker15
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sesker15 3/4/2014 9:08am
3/4/2014 9:08am
floridaflash wrote:
I would not run pump gas, if you are a fast, hard rider run race gas or at least a mix of pump and race, I...
I would not run pump gas, if you are a fast, hard rider run race gas or at least a mix of pump and race, I prefer race alone. It looks like it detonated, but sometimes it is hard to tell what happened first. I would split the cases to make sure all the pieces are out. make sure you put new crank and main bearings in it also while it is apart.
Oh cool. Thanks. I'm on it.
jsmx97
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jsmx97 3/4/2014 11:31am
3/4/2014 11:31am
Use new OEM crank, cylinder base studs and nuts, and cylinder head nuts and studs. As pete24 said, these yz's need atleast a race fuel blend even stock.

Make sure your gasket surfaces are SPOTLESS before reassembly and make sure they are no dents, high, or low spots on the gasket surfaces. check squish when reassembled. I like the wiseco YZ racers choice piston in the YZ's and rm250's.

Identify piston to cylinder clearance as well, although the replating company should do that when you send it out assuming it will need to be redone.
sesker15
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sesker15 3/7/2014 1:32pm
3/7/2014 1:32pm
Here are some cylinder pictures. It didn't get messed up.


sesker15
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sesker15 3/7/2014 1:35pm
3/7/2014 1:35pm
I think I found my problem. Bad reeds???

southwick_mx 3/8/2014 12:05pm
3/8/2014 12:05pm
Even seriously bad or broken reeds won't cause that, it'll just make the bike run crappy and first and foremost it'll be damn near impossible to kick start.
sesker15
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sesker15 3/21/2014 1:34pm
3/21/2014 1:34pm
southwick_mx wrote:
Even seriously bad or broken reeds won't cause that, it'll just make the bike run crappy and first and foremost it'll be damn near impossible to...
Even seriously bad or broken reeds won't cause that, it'll just make the bike run crappy and first and foremost it'll be damn near impossible to kick start.
Oh ok. Thanks for the help.
seth505
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seth505 3/21/2014 2:13pm
3/21/2014 2:13pm
I agree with others, I had an 09 yz250 from brand new til 2013 and never blew it up. Had good jetting but I used 1/3 race gas mixed with 2/3 93 pump gas it's entire life. I did Wiseco forged pistons at 40 hour intervals.
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Matt Fisher 3/21/2014 2:55pm
3/21/2014 2:55pm
Instead of race gas, just pick up some 100ll from the local airport. Even at 50/50 with 91 pump piss, it should be sufficient to keep detonation at bay.

Alternatively, have your head cut for pump gas. It might take away a little power (or might not if your squish is really big right now), but you should be able to run 91 pump without detonation issues.

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sandman768
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sandman768 3/23/2014 5:12am
3/23/2014 5:12am
agree with "pete24" I raced and had new yz 250 2 strokes from 99-2008. they need min of 50/50 race gas to correct pinging/detonation. I think in 06 Yamaha richened the needle slightly to help, but still not enough for east coast cool temps, sand or high rpm's rider's. I dealt with that for years, fmf pipes seem to make it run even leaner. Play it safe in cool temps, raise needle/lower clip to richen! At this point, pump gas is garbage everywhere, I run min, 50/50 in all my 2 strokes, if not 100% race gas. While you have cases split, inspect trans gears, for some reason, my last few yz 250's could barely go a full season on 1 transmission...
sesker15
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sesker15 3/24/2014 10:09am
3/24/2014 10:09am
sandman768 wrote:
agree with "pete24" I raced and had new yz 250 2 strokes from 99-2008. they need min of 50/50 race gas to correct pinging/detonation. I think...
agree with "pete24" I raced and had new yz 250 2 strokes from 99-2008. they need min of 50/50 race gas to correct pinging/detonation. I think in 06 Yamaha richened the needle slightly to help, but still not enough for east coast cool temps, sand or high rpm's rider's. I dealt with that for years, fmf pipes seem to make it run even leaner. Play it safe in cool temps, raise needle/lower clip to richen! At this point, pump gas is garbage everywhere, I run min, 50/50 in all my 2 strokes, if not 100% race gas. While you have cases split, inspect trans gears, for some reason, my last few yz 250's could barely go a full season on 1 transmission...
What were your jetting numbers on the 05 and up yz250 that you had for race fuel? And what race fuel did you run?
user760a
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user760a 3/24/2014 7:17pm
3/24/2014 7:17pm
that sure looks like a bunch of gritty dirty filthy oily sludge on the cases and crank (overhead view) suck a pound of dirt?
sandman768
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sandman768 3/25/2014 1:57am
3/25/2014 1:57am
Ok, I got my old notes out: In 2007 they richend needle taper, the needle is N3EW clip 2nd from the top in summer, 3rd in winter/cool temps or deep sand. Your stock needle should be N3EJ, look closely at the needle it will state part#. For race fuel, I would use VP c12 50/50 with 93 pump gas, if c12 not avail. then I would use Sunoco 110, same 50/50,as a local station near me had it at the pump. I can remember chasing a lean condition on the yz's whenever I rode the sand. listen to your engine, it will sound like crackling or marbles in top end when it's lean! that is your piston detonating itself to death! I would just go up 1 main jet size from the stock 178 to 180. . In the summer when its hot & humid, you can likely go back to 178, with richer needle, but I would run 50/50 race gas all the time! Better to start out rich & work your way back.

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