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4 stroke top end problem

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I was riding my 2009 KTM 450 EXC as usual the bike was perfect loads of compression it didn't miss a beat and started first time so later on on my way home I was riding up to a corner slowing down the bike cut out and it had lost half the compression and wouldn't start if anyone has any idea what it could be please comment any help would be appreciated.

I took the head off as I herd a clicking of the cam chain when I was riding, it all looked normal I Dont have a clue what the problem could be with the bike I began to take the top end apart to see what could be up it was all going well until I got the head off I was stupid an forgot to get the piston to TDC now the piston feels seized in the bore it wasn't before I dismantled it it moved freely now it has jammed I Dont know how to get it moving again any help would be great thanks

Edited by cr500clarkey

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First off, I would have attempted a compression test, or leak down test,  BEFORE I took the head off the bike. You have now put yourself in a position that it will be MORE DIFFICULT to diagnose the problem. For future reference, if you suspect you have a top end problem do a leak-down test, or compression test (have to disable decompression system if you have one) before anything else.

 

With that said, if the motor was not seized before you pulled the head, most likely you don't have the cam timed properly and the piston is now pushing against an OPEN valve that is supposed to be closed. The valve is open because you have the cam timing off. That would be my GUESS.

 

Do you have someone that has more experience working on these things that can give you a hand? That is where I would start. Either ask a friend for help, or find a shop you trust.

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How worn does the cam chain sprocket on the crank look?  If the bike has some hours, or the oil wasn't changed enough then that sprocket usually wears faster than normal and can lead to the timing chain slipping a tooth or two.  But you can only tell if this happened by checking the timing before you take it apart.  So remember that for next time.

 

At this point I'm confused; is the head back on and it's jammed, or is the head off and it's jammed?  If the head is off but the cam chain is still around the crank, check carefully that the chain isn't binding up around the sprocket.  Work slowly and carefully, it's easy to make a bigger mess if you're not careful.  Now would be a good time to take it to a mechanic if you're not feeling confident about it. 

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Thanks for all the advice when I dissasembled the top end everything was looking as it should I now think its a gasket causing the problem as the head gasket was torn and the seized piston was because the cam chain got caught up like you said but I now have a new problem to free the chain I had to turn the motor over its WAY out of time so I was thinking when I get the new gaskets and parts and assemble it would it be best to get a mechanic to time it for me ?

Thanks again for all the help

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