Valves



6 replies to this topic
  • kcracing

Posted April 07, 2014 - 08:47 PM

#1

Had the valves checked on my 08 yz450. Spec is .10-.15mm for intake and .20-.25mm for exhaust. Mine was .09 left, .04 center, and .09 right. Exhaust was in spec. Bike is supposedly low hour and looks it (20ish hours) according to previous owner. My question is why the .04mm center intake? Am I looking at a valve job? I just got the thing and Im worried about putting time on it in fear I'll be out of spec in no time.. Intakes are now set at .12mm

  • jrice2012

Posted April 07, 2014 - 08:56 PM

#2

Had the valves checked on my 08 yz450. Spec is .10-.15mm for intake and .20-.25mm for exhaust. Mine was .09 left, .04 center, and .09 right. Exhaust was in spec. Bike is supposedly low hour and looks it (20ish hours) according to previous owner. My question is why the .04mm center intake? Am I looking at a valve job? I just got the thing and Im worried about putting time on it in fear I'll be out of spec in no time.. Intakes are now set at .12mm


Just shim them. I'm in the exact same situation as you. Order a shim kit. You'll be good for many hours to come.

  • grayracer513

Posted April 08, 2014 - 06:59 AM

#3

The center always wears faster on a 5 valve head because the bulk of the fuel runs down the center port and removes the gummier elements of combustion residue that "lube" the other two.  Shim it and keep an eye on it.  If it will stay adjusted, it's OK.  If it tightens up more in under 5 hour run time, it needs to be replaced. 

 

On my '06, when this situation came up during a top end job, I replaced just the center intake (cut the seat, of course) and ran the rest as they are.



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  • kcracing

Posted April 08, 2014 - 07:21 AM

#4

The center always wears faster on a 5 valve head because the bulk of the fuel runs down the center port and removes the gummier elements of combustion residue that "lube" the other two.  Shim it and keep an eye on it.  If it will stay adjusted, it's OK.  If it tightens up more in under 5 hour run time, it needs to be replaced. 

 

On my '06, when this situation came up during a top end job, I replaced just the center intake (cut the seat, of course) and ran the rest as they are.

 

Thanks grayracer,  the mechanic that checked them advised on doing them all if it moved again. are there any downsides to doing just the center if needed?  I think the guy that sold it to me knew something was coming and got out while the getting was good. I'm just having trouble believing it on such a low hour bike.



  • grayracer513

Posted April 08, 2014 - 08:43 AM

#5

The downside is that there is an unknown amount of service life left in the rest of them that is shorter than what they had when they were new, so at some point, one or more of the rest of them will start sinking, and the head will have to come back off. 



  • kcracing

Posted April 08, 2014 - 01:42 PM

#6

The downside is that there is an unknown amount of service life left in the rest of them that is shorter than what they had when they were new, so at some point, one or more of the rest of them will start sinking, and the head will have to come back off. 

Historically from my own previous bikes and from what I have read Yamaha's tend to be pretty bullet proof in regards to valves.  If it was sucking dirt and my center intake is on the way out I'm really hoping to get away with just replacing it.  Did you have good luck with the rest of your valves on your 06 staying in spec after re-doing the center intake? As I mentioned my right and left were at .09mm which is not terrible but they were on the tight side.  Thanks for the info grayracer.



  • grayracer513

Posted April 08, 2014 - 02:58 PM

#7

So far, mine is just fine.  If you want to know about what's gone by the air filter, pull the top cover off the carb.  The air that runs to the hot start is routed through there, and there's always a kind of low level flow the runs through it.  The air comes from the carb mouth, right in front of the slide.

 

No air filter is perfect.  Clean and re-oil yours before EVERY ride day.







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