2010 Yamaha YZ450F Compression?

Just wondering how much compression my yz450f should have. Its a 2010 with 30 original hours, never rebuilt or had the valves adjusted. Seems to have low power compared to other 450's I've ridden and I'm thinking it either needs a rebuild or valve adjustment. Still starts fine and doesn't make any noises but is leaking oil out of the crank vent hose. I'm going to do a compression test but don't know what kind of numbers I should be seeing. Any knowledge would help, thanks.

Conventional compression tests are entirely useless on big single cylinder engines like these because, one, they have automatic decompression, two, they can't actually even be kicked over at full compression if the AD is disabled, and three, such tests are really only valuable on multicylinder engines as a means of finding one or more cylinders that are significantly lower than the others.  That's why Yamaha doesn't publish numbers.


What you need to have done is a "leak down test".  This feeds high pressure air into the cylinder and measures what percentage of the input pressure leaks away in numbers expressed as a percentage.  Some will always leak out, since the rings have gaps, but for a strictly recreational bike, it should hold over 90%.  A freshly broken in, race engine should test at over 97%.

Okay thanks a lot for the help

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