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Blown top end, why?

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Hey guys so I did my top end on my kx250 and I did the first heat cycle then realized that that my crank seal was blown so I took apart the bike and took the new piston out. Then I got new cranks seals bearing ect put the slightly use piston back in and then I started up the bike on the stand, it ran for a 5min and then it died I tried to kick it on but nothing, then I checked the compression it was at 90 and ya just never turned on. Btw I got the cylinder honed too. HELP ME!!

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Hey guys so I did my top end on my kx250 and I did the first heat cycle then realized that that my crank seal was blown so I took apart the bike and took the new piston out. Then I got new cranks seals bearing ect put the slightly use piston back in and then I started up the bike on the stand, it ran for a 5min and then it died I tried to kick it on but nothing, then I checked the compression it was at 90 and ya just never turned on. Btw I got the cylinder honed too. HELP ME!!

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Crank seals normally don't require crank removal .

Take the thing apart and the answer is there . All I can thing of that would kill it that fast is

Piston wrong size

Didn't hone properly

Or honed a nikasil

Didn't oil when putting together

Dirt!!

Piston in wrong

Really lean and burned out

Most likely it's a combo of these

TBH most of the time something that fast is normally either user error or something was worn out and didn't get replaced with the rest of the stuff

I'd just take it apart and see what sort of damage you got and be more careful rebuilding .

Who honed it ? How ? Did you measure everything ?

Btw if you honed a nikasil jug that would do it

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Crank seals normally don't require crank removal .

Take the thing apart and the answer is there . All I can thing of that would kill it that fast is

Piston wrong size

Didn't hone properly

Or honed a nikasil

Didn't oil when putting together

Dirt!!

Piston in wrong

Really lean and burned out

Most likely it's a combo of these

TBH most of the time something that fast is normally either user error or something was worn out and didn't get replaced with the rest of the stuff

I'd just take it apart and see what sort of damage you got and be more careful rebuilding .

Who honed it ? How ? Did you measure everything ?

Btw if you honed a nikasil jug that would do it

I dont know how but I got it running I just kick it about 100 times lol and it started right up and idle

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shut your face if you dont like it dont look at it

With all due respect there's a good reason for this rule and being hostile about it is a good way to get banned or have your posting abilities taken away

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shut your face if you dont like it dont look at it

You won't be posting much longer with this attitude. 

 

Read and follow or you're out of here.

 

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Cylinder honed or plated?

Piston in backwards...

it turned on I dont know how but it turned on and and idle shortly after that I had to shut it of because one of my lines poped off didn't fully tighten the hose clamp antifreeze got everywhere lol

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Backwards piston is possible.  If you put it in like that it might start and run just long enough for a ring to snag on a port since the alignment dowels would be in the wrong place. 

 

Take it apart again.

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Backwards piston is possible. If you put it in like that it might start and run just long enough for a ring to snag on a port since the alignment dowels would be in the wrong place.

Take it apart again.

its not backwards I put the arrow facing the exhaust

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Did you take it apart yet?  90 psi compression means you have a serious problem.  Either the rings are stuck or the cylinder wall is damaged.  Rings can get stuck if you have a foreign object in the cylinder that gets mashed against the cylinder head.  This can collapse the rings lands and stick the rings.  If something got into the motor while you had it apart that would do it.

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ImageUploadedByThumper Talk1453782535.680093.jpg

This was 130 psi. Something is seriously messed up to have 90

Edited by Yamahace

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