99 WR400 compression reading?

3 replies to this topic
  • creepy

Posted April 11, 2005 - 06:34 PM


Anyone know what this bike is supposed to show for compression? Ive got an erratic idle on a WR400 thats supposed to have been rebuilt by the previous owner. Im only seeing 100psi cold, which sounds low for 12.5:1
Ive got an 'owners service manual' which doesnt list a compression spec.. is this different from a shop manual?
TY in advance guys =)

  • C-ya

Posted April 11, 2005 - 09:35 PM


A shop manual will have what you want. Mine is at home right now. If no one posts back with the comp., I'll do it when I get home. I can't remember if I have even looked at what it is on this bike ( I have a '99 400 as well), but my old XR200R was around 140 or 160 psi.

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  • C-ya

Posted April 12, 2005 - 06:46 AM


OK, got the manual in hand. I quote: "Yamaha does not provide a required compression specification for this engine, however, an engine of this design typically has 170-180 psi (1195-1265 kPa) compression when broken in properly." It goes on to say that one should take periodic readings to have something to compare to should you actually have a low reading.

  • creepy

Posted April 14, 2005 - 10:12 AM


yup that sounds more like it. Ill check my valves first, otherwise it sounds like the previous owner goofed something


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