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Everything Dirt Bike
Make / Model Specific
Yamaha
1978 YZ400 Piston Seized
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<blockquote data-quote="aoyue2702" data-source="post: 133756" data-attributes="member: 1291"><p>So... I was riding down the road breaking in the new piston and freshly bored cylinder today, was going about 55 mph on the pavement in 5th gear. All the sudden the engine dies and the rear wheel locks up causing me to go sliding left and right until I thought to pull the clutch in. Pulled over to the side of the of the road, put it in netrual, and it started first kick.</p><p> </p><p> </p><p>I went back to the house and I tore the top end apart again.</p><p> </p><p>Here's what I found. Two corners appear to have seized on the piston, and the other two corners appear to look nearly brand new??? What could have caused a 2-corner seizure? The cylinder looks to be in better shape than the piston, however two corners of the cylinder does have some vertical scratches and scuffs in those spots.</p><p> </p><p>Is there any way I could possibly sand this or something along those lines and run it for the summer? (I know, crappy way to go about things, but I really, really dont have another $200 to drop on a fresh top end again.) Also, I noticed the top of the piston had little bitty specks all over it, and when I looked a bit closer, it appears to be micro chips? What could have caused this?</p><p> </p><p>The jetting in the manual calls for a 370 jet, however, I put a new carb on it and it's got a much smaller jet (I think a 320 or 330), would that have caused such a seizure? The bike also has a header leak and is missing the muffler.</p><p> </p><p>There was almost no vertical play in the connecting rod, The bike was almost warmed up fully, probably been on for maybe 3-5 minutes</p><p> </p><p> </p><p>Heres some pictures</p><p> </p><p><img src="http://i33.tinypic.com/2qcnipd.jpg" alt="" class="fr-fic fr-dii fr-draggable " style="" /></p><p> </p><p><img src="http://i36.tinypic.com/w1d9x1.jpg" alt="" class="fr-fic fr-dii fr-draggable " style="" /></p><p> </p><p><img src="http://i36.tinypic.com/2r5ysuu.jpg" alt="" class="fr-fic fr-dii fr-draggable " style="" /></p><p> </p><p><img src="http://i37.tinypic.com/11qpcee.jpg" alt="" class="fr-fic fr-dii fr-draggable " style="" /></p><p> </p><p><img src="http://i35.tinypic.com/v4z614.jpg" alt="" class="fr-fic fr-dii fr-draggable " style="" /></p><p> </p><p><img src="http://i38.tinypic.com/27y33nb.jpg" alt="" class="fr-fic fr-dii fr-draggable " style="" /></p><p><img src="http://i33.tinypic.com/14c7i1e.png" alt="" class="fr-fic fr-dii fr-draggable " style="" /></p></blockquote><p></p>
[QUOTE="aoyue2702, post: 133756, member: 1291"] So... I was riding down the road breaking in the new piston and freshly bored cylinder today, was going about 55 mph on the pavement in 5th gear. All the sudden the engine dies and the rear wheel locks up causing me to go sliding left and right until I thought to pull the clutch in. Pulled over to the side of the of the road, put it in netrual, and it started first kick. I went back to the house and I tore the top end apart again. Here's what I found. Two corners appear to have seized on the piston, and the other two corners appear to look nearly brand new??? What could have caused a 2-corner seizure? The cylinder looks to be in better shape than the piston, however two corners of the cylinder does have some vertical scratches and scuffs in those spots. Is there any way I could possibly sand this or something along those lines and run it for the summer? (I know, crappy way to go about things, but I really, really dont have another $200 to drop on a fresh top end again.) Also, I noticed the top of the piston had little bitty specks all over it, and when I looked a bit closer, it appears to be micro chips? What could have caused this? The jetting in the manual calls for a 370 jet, however, I put a new carb on it and it's got a much smaller jet (I think a 320 or 330), would that have caused such a seizure? The bike also has a header leak and is missing the muffler. There was almost no vertical play in the connecting rod, The bike was almost warmed up fully, probably been on for maybe 3-5 minutes Heres some pictures [IMG]http://i33.tinypic.com/2qcnipd.jpg[/IMG] [IMG]http://i36.tinypic.com/w1d9x1.jpg[/IMG] [IMG]http://i36.tinypic.com/2r5ysuu.jpg[/IMG] [IMG]http://i37.tinypic.com/11qpcee.jpg[/IMG] [IMG]http://i35.tinypic.com/v4z614.jpg[/IMG] [IMG]http://i38.tinypic.com/27y33nb.jpg[/IMG] [IMG]http://i33.tinypic.com/14c7i1e.png[/IMG] [/QUOTE]
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1978 YZ400 Piston Seized
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