bike has no compersion

6 replies to this topic
  • ride yzf all the way

Posted September 02, 2005 - 04:47 AM


i have a new 05 yz 450f and it had no compression starting it the other day so it wouldnt turn over just wondering what might if caused it cause now its fine

  • sirthumpalot

Posted September 02, 2005 - 05:15 AM


Has happened to many of us, not 100% sure why. Best guess is something washed the oil off of the cylinder and the rings don't seal, then once it fires (or is kicked enough) the oil returns and all is back to normal. I can make it happen on my 426 by shooting carb cleaner into the carb while it's still on the bike. Presto no compression, but it fires and is back to normal immediately after it fires.

  • Daz_426_02

Posted September 02, 2005 - 05:15 AM


Might have been somthing to do with the auto decomp lever on the cam... not to sure why it it would work then not work, then work again ?? strange one.

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  • DaveJ

Posted September 02, 2005 - 10:51 AM


Same problem on just about all of mine. Once they warm up, everything is fine.

  • grayracer513

Posted September 02, 2005 - 01:56 PM


Try getting in the habit of rolling the engine up against compression when you shut it off and put it away for a while. This will prevent atmospheric moisture from getting at the valve seats and possibly rusting them, which can cause that problem.

It's also possible to have a flake of carbon fall into the valve seat, or to have a valve stem stick in the guide and not close all the way.

The decomp mechanism cannot cause this, although it can cause the oppsite effect; if it sticks in the "run" position, it won't relieve compression when starting.

You should also check your valve clearances.

  • 99raptor

Posted September 02, 2005 - 02:54 PM


Great idea "grayracer" about bringing it up on compression when your done for the day.Now if I can only remember to do it.

  • Chills

Posted September 02, 2005 - 03:02 PM


Hey Gray, what if the decompression is locked into the starting position. That would cause it to open the exhaust valve with each rotation, thus not allowing any compression to build. Then after awhile of kicking it over, it finally unseats itself, and retracts like it should...

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