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supaflyrt

1998 YZ400F

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Purchased a non-running 1998 YZ400F, engine would rotate until very nearly TDC and stop. Disassembled the top end and found the right intake valve had broken and became lodged in the top of the piston. The cylinder somehow didn't sustain any damage. On disassembly I noticed it has a hot cams intake cam in it. The top of the piston and cylinder head were dark like it was running a tad rich maybe, no sign of overheating. Are these bikes prone to valve failures?

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Only when neglected.  The previous owner may have simply run it too long after the valves had significant wear on them (adjusting them beyond .10mm thinner than stock shims isn't advisable).  The black carbon means nothing at all about the jetting, BTW. 

 

The head can probably be repaired by a shop such as Engine Dynamics, but with new valves and such, it may be close to $500. 

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Thanks! Will probably try my luck with a used head off eBay though, can't seem to find anything local (Alberta, Canada). Do you happen to know if any newer heads will work or am I limited to 98/99?

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