Possible problem auto decompression 08 YZ450

6 replies to this topic
  • Phil Hannah

Posted January 18, 2008 - 02:46 PM


I dont know if this is a trait of the YZF, but sometimes it seems to get on compression stroke and you cant kick it over and sometimes it feels like the auto decomp sticks and you have no compression.I pulled valve cover and auto decomp assy seems fine. The bike has 2.3 hours on it. I have been racing for over 25 years and this is my 5th four stroke, but my first YZF.Any input would be appreciated.

  • grayracer513

Posted January 18, 2008 - 03:18 PM



  • Phil Hannah

Posted January 18, 2008 - 03:50 PM


Thankyou for the reply but I cant pull it up because something is up with my computer and I have not a clue! Is there some way you can post it in its complete form or just the most important info that can help,please?

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  • Way Fast Whitey

Posted January 18, 2008 - 03:52 PM


The occasional failure to decompress properly is generally associated with a stall condition. If the engine stalls against compression, which it's actually pretty likely to do at low rpm under a load, it may come to a stop so abruptly that the decompression mechanism doesn't have a chance to reset itself before the engine stops. This will trap the decomp pin in the retracted position, and the flyweight return spring simply isn't strong enough to force the exhaust valve off its seat. The cure is either to slowly push it through compression, or to put it in gear and rock the bike backward away from the compression stroke, allowing the auto decompressor to fall back to its starting mode.

If, OTOH, the bike is now consistently very hard to turn over, and it's just since the work was done, there may be a problem with the auto decompression unit, or the timing of the exhaust cam. If that's the case, you should talk to the service manager

greys answer to basically the same question.

  • Phil Hannah

Posted January 18, 2008 - 04:05 PM


Thankyou for the reply. I have had 3 hondas and one rmz and never had this problem. This could be a problem when stalled in the heat of battle,which I still tend to do! I wonder if there is a mod that can be done to auto decomp mech that might help?

  • grayracer513

Posted January 18, 2008 - 09:29 PM


You stall it in gear, and "in the heat of battle", you probably try to kick it while it's still in gear, right? Let the clutch out and pull the bike back 3 inches and you're clear.

There's nothing you can modify to fix this, although raising the idle may help, and Hondas do this, too.

  • Phil Hannah

Posted January 19, 2008 - 07:22 AM


Good morning Mr.Gray. No I never try to start it in gear,but now I will try to add rolling the bike backwards to the procedure. I always was happy just to find neutral! I havent compared compression ratios,but it seems like this bike has more than my other 450s.Thankyou once again for the reply.

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