1 reply to this topic
  • SM421

Posted November 28, 2009 - 02:55 PM


OK here's a question for you guys with engineering degrees. I've owned an '04, '06. and '09 YZ450F. None seemed to have the kind of torque that my '99 YZ400 had. Why would a smaller displacement engine seem to have more bottom end power?

The other thing I noticed is that all the 450's have the popping on deceleration even with rejetting. The 400 never did.

Don't get me wrong, the 450's are better in the handling department than the 400 ever was but I do miss the tractor like power.

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  • grayracer513

Posted November 28, 2009 - 03:17 PM


You don't need a degree for that. The greater low speed power comes from milder cam timing and a relatively larger amount of rotating mass inertia.

Insofar as the decel backfire goes, that too is partially due to aggressive valve timing, but can also be due to an air leak in the exhaust or a lean pilot circuit. A certain amount of it should be considered normal for a YZ450, and attempts to eliminate it entirely by richening the idle circuit usually result in a too-rich idle.

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