New Theory on intermittent Compression loss

7 replies to this topic
  • Frostbite

Posted April 14, 2005 - 09:33 AM


I'm thinking now that my random compression loss may be caused by a valve bucket hanging up in it's bore. Mine is probably caused by icing, snow sucked into the breather hose and frosting up the buckets. This would explain why the bike runs better when hot, the ice melts. For you southern boys you may have sucked in some crap that caused a bucket to momentarily hang up holding a valve open.
I've eliminated rings since this happens and cures itself suddenly, so that leads me to valves. My valve clearances are correct and it's not a hot valve tightening up because on the weekend it happened while I was trying to start a cold engine.
I'm going to reroute my breather hose to the airbox and see if that cures the problem.
You boys that had similiar compression loss, did any of you have the breather rerouted or was it factory by the peg?

  • YZ250F_Rider

Posted April 14, 2005 - 09:42 AM


check that you dont have a build up of gunk on the back of the valve faces. This can easily lead to hanging valves at low revs like kicking.

  • Frostbite

Posted April 14, 2005 - 09:44 AM


It's also happened 2ce at full throttle, slow compression loss, then died, then compression came back and ran fine. :naughty:

  • YZ250F_Rider

Posted April 14, 2005 - 10:08 AM


It still can be hanging valves at wot too. it's just that a hanging valve is about the only thing that can cause intermittent compression loss at slow rpms.

Are you sure it was a loss of compression at high speed and not a fuel starvation.

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  • Frostbite

Posted April 14, 2005 - 10:42 AM


Oh yeah, it felt like a 2 stroke melting, slowed down and then died. When I tried to restart it felt like the sparkplug was out. I could push the kick lever by hand. Then full compression came back a while later and the engine was back to normal.

  • grayracer513

Posted April 14, 2005 - 11:06 AM


check that you dont have a build up of gunk on the back of the valve faces. This can easily lead to hanging valves at low revs like kicking.

In particular, look at the valve stem, at the point where it meets the guide, and at the margins of the valve face at the inside diameter of the ground 45 degree face of the valve, at the edge of the back of the valve head. Carbon at either location can hold a valve off, then chip away and let it seat, then return again.

  • Frostbite

Posted April 14, 2005 - 11:14 AM


Greyman, do you think it's possible that something could hang the buckets up?

  • R_Little

Posted April 14, 2005 - 11:37 AM


It's happened to me once when the bike was brand new.

No compression. Could kick it by hand w/o compression release.

Panic set in

Walked away kicked it again, it started and all was well.


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