YZ450F Spitting oil out of the cylinder?

6 replies to this topic
  • shiftngo

Posted February 18, 2009 - 12:47 PM


My 04' 450 is spitting oil out of the front of the cylinder where i guess a manual decompression thing would go. But seing as these are auto-decompressing bikes, I don't know what to do to fix the problem. Is there supposed to be a bolt or something in there to keep this from happening? It spits oil directly onto the radiotor and is constantly smoking from the oil that gets on the exhaust. It's not a whole lot of oil, but enough to annoy me and make my YZ look BoBo when I fire it up. Please help!

  • KJ790

Posted February 18, 2009 - 12:53 PM


There is a plug that goes in that hole on those bikes. Sometimes that plug can pop out, especially if the vent tube to the head gets pinched off. You need to replace the plug. There are aftermarket ones available on ebay (they used to be for sale through the Thumpertalk store, but I think they stopped selling them) that stay in better than the stock plug.

  • shiftngo

Posted February 18, 2009 - 03:17 PM


thank you very much. It's been driving me crazy and I looked at OEM diagrams but couldn't see anything that was supposed to go there.

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  • shiftngo

Posted February 18, 2009 - 03:18 PM


Oh, and do you know what it's called?

  • grayracer513

Posted February 18, 2009 - 04:38 PM


It's called a decompression bore plug. The original item is listed on the Camshaft/Chain page of the parts book.

My favorite substitute for it is an 18mm automotive cup plug (freeze plug, expansion plug, etc.)

  • KJ790

Posted February 18, 2009 - 04:41 PM


Here is the aftermarket plug:

The stock plug is not listed on the Yamaha parts diagram with the head for some reason. It is with the camshafts. It is #15 in this diagram:
Posted Image

  • Crazy Horse

Posted February 22, 2009 - 02:22 AM


I sure am glad I logged in and looked through some posts before hauling my bike in....its been spitting a small amount of oil on the exhaust as well and this sounds like the likely culpret

thanks a million:thumbsup:

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