99 WR400 compression reading?

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 =)


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.

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.

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

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