98 wr400f. Such a thing as too much compression?

4 replies to this topic
  • KIDO81

Posted September 25, 2013 - 03:21 PM


I brought a 98 wr400 and the guy said that the 2 intake valves might need a shim!!
I took the motor out of the frame and used a gauge. I could fit a .07 on all except 1 intake.
My question is if I try to use the kick to start it I could only do one kick and they I have to use the decomp valve to release the pressure and then I could try to kick it again. Would 1 valve make such a difference???
Sorry I'm new to the 4 strokes always had 2 stroke.

  • grayracer513

Posted September 25, 2013 - 03:35 PM


Correct valve clearances for a YZ/WR400 are .10 - .15 mm and .25 - .30 on the exhaust. 


Having a valve too tight will have the complete opposite effect; it lowers compression.  Nevertheless, a healthy WR400 will REQUIRE the decomp lever in order to be started.  Normal compression should all but prevent it from being kicked over otherwise.


  • Push the kick crank through until it stops against the compression stroke (the "hard spot")
  • Pull the decomp lever and move the starter lever downward from one to two inches (no farther)
  • Release the decomp, reset the kick crank to the top, and kick through

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  • ShsFunny

Posted September 26, 2013 - 11:03 AM


I just put a new piston in my yz 400 f. Thought that was all that was wrong. Had massive compression while rebuilding. Put the valve head back on, poof no compression. The head is in the shop now for a complete valve job + seat cut job. If I can't stand on that kicker when I get it back, I WILL set it on fire.

  • ggggg

Posted September 26, 2013 - 11:39 AM


Sounds like you have good compression, but get those valves shimmed right, & keep an eye on them to make sure they are not closing up the gaps or you could end up with problems.

  • Thommo_400f

Posted September 26, 2013 - 09:41 PM


If you DON'T have to use the decomp lever, it means you don't have enough compression.

That's why it's there  :thumbsup:

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