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Valve spec question

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Hey guys, 

 

I tried the manual links in the FAQ but they didn't work and I tried searching but couldn't find any.

If anyone can send me a working link that would be cool or just post up the valve clearance specs for a 2007 YZ250F.

I am trying to get a friends bike running for him and want to check out the valves. He doesn't know the last time it was done and he is complaining of hard starting.

 

Thanks in advance.

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Ok, so here is my next question. What is the minimum thickness of shim that can be used?

The intake valves were so tight that I couldn't fit any feeler gauges under the cam lobe. I used 0 as my starting measurement to determine the next shim to use (present shim size + measured clearance - desired clearance = new shim size)

After the first round of adjustment, the left side was in spec at .13, the center was still at 0 and the right side was at .08. 

I re-adjusted the right and center and got them within spec (center .15, right .13) but for the center I used a 1.25 shim. I am concerned that the bucket might be pressing directly on the retainer. is this possible? 

 

I know the intake valves and seats are going to need replacing soon as that was a huge adjustment and its down to some of the smaller shims that I have. I could tell that they were adjusted before since I saw a hot cams shim already in it The exhaust valves were still in spec so I'm thinking that the excessive wear on the intake side is from dirty air getting through the filter but wouldn't dirt getting past the intake valves also cause the same excessive wear on the exhaust side? What else would cause wear on the intake valves and not the exhaust? He is using a K&N and I already suggested that he goes back to good old fashioned foam filters when he rebuilds it.

 

Any way the meter on the bike shows over 200 hours so if the meter is correct the head definitely lasted longer than I would expect. Ive heard a lot about yamaha durability but that is awesome. I may have to switch brands in the future is that is the service life of an engine without any major rebuilds/regular valve checks.

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You have several questions in there!

 

Essentially, the intakes are always going to be the valves that go out of spec first (and usually the center one will move faster than the outer two) as compared to the exhaust valves, which rarely, if ever go out of spec first or at all.

Typically, the best thing to do (knowing that seats and valves are going to need replacing) is to just replace them all as a set on the newly cut seats.

But since you mentioned a 1.25 shim being used on the center intake to get it in spec, a new head is in order and the bike should not be ridden with a pad number that low. That is just begging for a dropped valve causing a catastrophic result.

I myself, won't chance going below a pad range number of say around 1.70 ~ 1.75 on a second adjustment. A good rule of thumb is to not adjust the clearance more than twice before replacing the valves.

When you have a shim that low (1.25) there is nothing left to remove from the seat to put the head back in usable service.

Now I know cylinder heads are not cheap, but then again, we are talking about a 2007 and someone may chime in if they know of someplace that could just replace the seats if that would be cheaper than buying a whole new cylinder head.

When you mention over 200 hours on the meter, it is not uncommon for valves to last that long with proper maintenance. Knowing the history of the bike outside of hour meter terms is a big plus as well.

As far as the bucket possibly pressing directly on the retainer - sure, if that stub inside the bucket is worn down, I guess it might be possible; but then again, I don't know too many guys that will run a shim size that low unless they can walk on water!

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Rule of thumb: whenever any titanium valve needs to use a shim two full sizes smaller than the one it was originally built with ( -.10mm or .004"), that valve should be replaced, as it indicates that the hard coat on the valve face has worn through.  It will also be true that the valve will not stay adjusted for more than just a few hours once this happens.

 

Valve seats in YZF heads rarely need replacing unless the head has been severely overheated, causing the seats to distort out of round, or there has been some catastrophic damage, such as dropping a valve head, or, unless the engine has been run for a very long time with very worn valves.

 

At 200 hours, another possible explanation exists, and that is that the valves have already been replaced once, and the seats necessarily reground along with that.  Such a situation creates a condition in which the starting shim with a brand new valve may be significantly thinner than the original to begin with.  I've had heads under repair that needed to get set up with shims as small as 140's to fall into spec with new valves, so someone opening that for an adjustment down the road would find it necessary to use a relatively thin shim for the purpose.

 

The lifters should not come into contact with the retainers with #125 shims, but running with badly worn valve faces causes accelerated wear to the seats, and can result in a "scrubbing" sort of dynamic at the valve head as it closes because the wear at the valve may not be concentric.  That scrubbing can lead to fatigue at the joint of the stem and valve head, and it can break off as a result. 

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Eloquently put grayracer513!

 

At 200 hours or more, it is possible that the valves have already been replaced once and the seats reground.

 

Thanks for the clarification I wanted to point out a little more clearer, but it eluded me this morning.

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Low caffeine levels?

Yes - I ran out of coffee yesterday and didn't get it until about noon today. I don't drink pop or soda enough to always have a backup source of that liquid mental fuel :goofy:  .

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Kickstand and Gray Racer, 

 

Thank you guys for your input on this topic. I am still building my knowledge on maintaining 4 stroke heads so I appreciate the info.

 

The guy came by and picked it up earlier and I pretty much told him what you guys are recommending. I told him that it will run but its not a good idea to push it much longer and suggested a rebuild of the head soon and talked about the possibility of a major failure due to the condition of everything (dropped valve had crossed my mind). I think he is going to try to get everything done before the next local XC race so he has some peace of mind while racing. He just got the bike a month or so ago so its a good idea to freshen everything up.

I had Fast Heads re-cut my seats before since it was the first valve job on my bike but they are supposed to be able to reinstall new seats and valve guides in worn heads. Hopefully that can save a few bucks since a new head that I bought previously for another bike was $500 bare.

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