08 yz450f, new to me

5 replies to this topic
  • brendan92298

Posted May 12, 2015 - 09:13 AM


My brother just bought a used 08 yz450f the guy he got it from said he had to do head work because it jumped timing. He test rode it and it looked like it ran fine got through all the gears no prob. But we went to the dunes and it was a little poppy slowing down, he let me take it for a spin and it felt super dead on the low end, mid and top were powerful though.
Do these bikes have problems with valves like the crf's?
Is it normal for this bike?

  • grayracer513

Posted May 12, 2015 - 09:35 AM


No valve problems like the older CRF's.


If the '08 is completely stock, yes, they normally feel a bit sluggish at low RPM.  There are two reasons for this.  One is the cute little "shorty" muffler together with its oversized header.  A full system is the best cure, even a full stock system from an '06 or '07, but even a decent slip-on will often add as much as 5 HP at 5000 RPM.


The other is the timing map in the stock CDI.  Switch to a CDI from an '09 or an '06 instead, and the bike will wake right up.


Popping on deceleration is either causes by an air leak at the header or mid pipe (always check that first) or having the idle too lean. HOWEVER, a YZ450 in good health with the right jetting should pop "some" on deceleration.  Just their nature.  Not continuously all the time, all the way to a stop or anything, just some popping some of the time.

  • brendan92298

Posted May 12, 2015 - 03:41 PM


Thank you gray racer for the insight

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  • n16ht5

Posted May 13, 2015 - 04:55 AM


take the shorty muffler apart and ram a 30MM socket through the front ice cream cone, that cures most of the power loss. Otherwise I have an 07 complete system with a PMB end cap I'd send you

  • grayracer513

Posted May 13, 2015 - 06:16 AM


The trouble with the approach above is that the bike gets to be, um, LOUD, and the muffler still doesn't let the engine breathe as it should.  But it is an improvement (powerwise).

  • n16ht5

Posted May 14, 2015 - 07:47 PM


The trouble with the approach above is that the bike gets to be, um, LOUD, and the muffler still doesn't let the engine breathe as it should.  But it is an improvement (powerwise).


I thought this would be the case.. actually it is still quieter than an 06 pipe. I put fresh packing in it and it isn't noticeably louder than before the front ice cream cone.

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