2008 Yz450f No Comp?

2 replies to this topic
  • motomanwr250

Posted August 21, 2011 - 07:07 AM


The other day I went down to start my 2008 Yz450f and it had no compression when i tried to find tdc to start it. I was able to continually cycle the engine easily by hand, that is how little comp it had. The weird thing on this is that it ran mint and crisp two weeks before and had not been touched since. (started in one to two kicks, great throttle response.)

Any Ideas on what this could be before i start tearing the engine apart to find the problem?

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  • grayracer513

Posted August 21, 2011 - 07:25 AM


Start by checking the valve clearance and timing. You may have sunk an intake.

The thing that can sometimes happen is this: In spite of what most people intuitively think, valve clearance in a YZF increases as it heats up, not the other way around. This is because the head expands more than the valves do, moving the cam away from the valve seat. When a valve at normal operating temperature wears down to near zero clearance, it actually will be held off its seat when it gets cold; less than zero clearance.

OTOH, you may have a valve being held off its seat by carbon, or by a rusty seat (I hear it's been nasty hot and humid up your way). If this is the case, you'll find one or more valves with too much clearance. Gently tapping the valve lifter downward a few times may bounce the valve back onto it's seat again and restore compression.

  • motomanwr250

Posted August 24, 2011 - 09:36 PM


Thanks for the advice i went to check clearances and noticed that one of the buckets was stuck down and an exhaust valve was stuck open. Took off the top end, problem solved valve guide had some rust on it from moisture causing the valve to stick. Cleaned everything up and its good to go.

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