98 YZ400F Decompression problem

2 replies to this topic
  • Twoply

Posted January 27, 2008 - 05:06 PM


Hi everyone!
New to the forum and to the Yamaha thumpers.

I bought a 98 yz400f last week. It's definitely 10 years old but it runs strong and they are tanks from what I hear.
Problem is, There is no difference between when I use the decompression handle before starting or when I dont. I think it's not properly working, but I've not a lot of experience with this. I havent taken it out yet to see if there is a loss of power.

I have the detail of the top end of the engine, but there isnt a lot of description as to how it's put together.
I'm the kind of guy who'll figure it out when I do tear it apart, but any advice is welcome. Is this common?

Thanks in advance all!

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  • jbrooks26

Posted January 27, 2008 - 05:56 PM


It may have a newer automatic decompression exhaust cam installed. The only way to tell for sure would be to pull the cam cover and look at the sprocket on the exhaust cam. It will be the front one, and if it has been upgraded then you are miles ahead of the game. It will have a small counter weight in the center of the sprocket if it is an autodecomp cam. However, if it is not then you have a compression leak somewhere that you will need to find. If it is a stock cam then you should not be able to kick through the compression stroke without pulling the decomp lever. Hope this helps.


  • grayracer513

Posted January 27, 2008 - 08:26 PM


With either the original cam or an Auto Decompression cam, there should be a distinct "hard spot" in the rotation of the engine as it reaches the compression stroke. Pulling the compression release lever should make it go away, so that the engine will turn freely with no detectable compression at any point. If pulling the lever does not do this, it may be poorly adjusted.

If there is no detectable compression when you turn it through, then you have a bigger problem.

As JBrooks points out, it may have an AD cam in it now. If so, it will look like the picture below. If it's still stock, both cams will look like the intake at the right.

Posted Image

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