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Yz250 keeps blowing ughhhh

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03 yz keeps keeps blowing the top end first 2 times it went I rebuilt it last time l...bottom and top end was rebuilt by a shop only has 9 hrs on new top and bottom....I always mix gas 32:1 with 93 octane...90% of time I had a little more oil....could it be the pistons that I use which is wiseco...which I doubt...r do I need to try a cast iron piston to see if I get a different outcome...I'm confused don't know what to do ...I love the bike was thinking about selling cause I can't deal with the problems anymore

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First off, what do you mean by "blowing the top end"? Need some details, what exactly is happening? And whatever is happening, it has a source besides the Wiseco pistons. I've got over 70 hours on the Wiseco in my '03, no problems.

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Haven't broke it down it down yet...but it killing pistons...like I said haven't broke it down yet but it lost compression

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Bet you have a lean condition and the piston has a hole. 

Let us know with photos and how the bike is set up. From the reeds, to jetting, to all settings. 

There has got to be some combination that is not right. 

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11 hours ago, cordlaw said:

I'm confused don't know what to do ...I love the bike was thinking about selling cause I can't deal with the problems anymore

You are killing pistons because you're not fixing the problem.

If you sell it, then someone gets a bargain bike.

What do the previous damaged pistons look like?

 

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Repetitive seizures could be caused by throwing in new pistons in a worn out or damaged cylinder,

an air leak that allows the engine to run lean (ever do a crankcase leakdown test?) how old are the crank seals ?

On re-assembly, do you measure the bore and match the piston size, measure/adjust the ring end gap to specs or just slap new parts in straight out of the box?

Was the bottom end checked at all before re-assembly. That could be full of debris and messing up your new piston the moment you re-start your 'rebuilt' engine.

Post clear pics of each side, top and bottom of the piston, cylinder etc.

 

Edited by mlatour
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So I have it broke down cylinder is still good ...thank god20170326_140630.jpg20170326_140622.jpg20170326_140649.jpg20170326_140644.jpg

Detonation. You are lean by a mile. Cylinder also needs replate. No debates needed, just send the cylinder to millennium, tell them to size it then call James Dean and give him the info on the bike so he can send you a jet kit.
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And for godsake clean your &%$#@!ing bike before you work on it. You go to the butt doctor without wiping your ass?

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I would say that is detonation. If so theres a big chance you are running lean. I would definitely open up the carb and check what jets are in there. Also what spark plug are you running? Also get some more pictures of the piston like close ups and of the skirts as well.

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178 main jet don't have a screw driver small enough to get the smaller one out...br8eg ngk plug

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