blown midvalve?

10 replies to this topic
  • luvmythumper

Posted November 24, 2001 - 08:09 AM


I am very new to doing any work with the forks. I will be changing the fluid today. I have read that there is a problem on the 426 with a blown mid-valve. Is this easy to check? How can I tell if the mid-valve is okay.


  • Big_Bang

Posted November 25, 2001 - 04:37 AM


The midvalves are an overstated problem. Very few of the ones I've looked into were actually bent. When MXA says something ....well, everyone thinks they too have the problem. If your forks have a tendency to dive through travel,ride lower then other bikes(providing you have the correct spring rates) and bottom easily over most obstacles, then you might be a candidate. If turning in the compression adjuster does nothing but yield a harsher ride with continuous bottoming is another clue. The fix is not bypasssing either. Find a shop that changes the midvalve stack not replaces it with a check valve.

  • MX_Tuner

Posted November 25, 2001 - 04:59 AM


Personally, I've had excellent success disabling the midvalve. Putting all the compression damping duties on the compression piston isn't a bad idea. Then I never have to worry about it again.

  • holeshot

Posted November 25, 2001 - 05:30 AM


The tuners at RG3 say the same thing - the "blown mid-valve" deal is not very common. If your over jumping 120 foot doubles, there is a slight chance the problem may arise.

  • DaveJ

Posted November 25, 2001 - 09:41 PM


I lost compression on my forks three times, before I pulled the mid-valve out and replaced it with a check plate.

However, I was never able to verify if the compression loss was due to a faulty mid-valve or seal head (cylinder valve). Both have since been modified or removed with no re-occurring problem.

In addition to this, upon close inspection, I was never able to site or feel any difference in the mid-valve shim stack when my problem occurred. Unless a fatigue shim(s) is only noticeable via other means, I'm not too sure where this whole blown mid-valve thing came from.

I had also spoken with the guys at KYB about the mid-valve, more on the issue of the harsh ride than failures, and their take was to remove it. However, I don't think they understand that application concerns involving the seal head.

If I had to put my chips on something, my guess would be that the mid-valve never fails, and it's the seal head that is causing all the problems. Based on what I have read, the dirt rag guys don't even know what the difference is.


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  • John_Curea

Posted November 26, 2001 - 05:58 AM


"Blown" seal heads...

That description sounds pretty horrendous...actually a "blown" midvalve is nothing more than a distorted face shim on the compression side of the active piston. The problem arrises when the face shim gets distorted it looses it ability to provide damping.
A shim provides damping (spelled resistence)by restricting oil flow due to the properties of spring steel. When the shim is distorted or bent it cannot provide enough resistence of the oil flow.
In 2001, Kayaba tried fixing the problem by increasing the "float" of the midvalve compression shimstack.They increased the length of the spacer from 6mm to 7mm. The "float" dictates when the midvalve becomes effective. A midvalve with less float will be felt at a slower shaft speed than a midvalve with a geater float.
Unfortunately, when they did this , they made the midvalve less effective.

The biggest issue I can see with the removal of the midvalve compression shim stack and installing a checkplate will be the loss of "midspeed" damping. When all of the damping is done on the base valve it usually results in a compromise.
If you valve for lowspeed, the high speed damping suffers (because the stack is too stiff) and if you valve for the highspeed hits, you give up too much low speed damping. An effective midvalve will "bridge the gap". You can build the base valve to be supple on the high speed hits and have the midvalve supply good bottoming damping.

I agree, that a dirty cylinder valve will cause a problem, that might look like a blown midvalve. Truthfully, all of the late model Yamahas that I have worked on have had the midvalve face shims distorted to some extent.
The cylinder valve has inward deflecting shims, they are designed to open upon a high speed hit.
If they are contaminated, they will leak and cause a problem. Since they are part of the high speed damping circuit, if CVs are removed, the high speed stack on the base valve should be adjusted.

Take Care,

  • DaveJ

Posted November 26, 2001 - 08:37 AM



Good post.

As for the opinion of removing or keeping the mid-valve, I think the design in theory is good, and perhaps in application when the fork is finely tuned and maintained. But for durability and longevity, the factory configuration doesn’t seem to cut it. I think the degradation in performance is too extreme for most of us pounding these bikes on a regular basis. It’s also seems to be a nightmare for tuning when you don’t have a constant environment in which to work with. It’s like to trying to jet an engine that changes cam timing every few hours.

Ideally, if KYB perfected the design, making it more durable, consistent, and serviceable, I’d leave the unit in, but I have had too much success otherwise. Do I recommend that others pull it out? Not really, because the rest of the design has to be changed too much to make up for the difference. Although most of time I have spent on these forks was trial and error, my current design may be applicable only to my weight and riding style. So I usually stay low key with the information.

So again, good post and I think it’s great for the reader here to see the opinions of others. I get a little chatty sometimes, so I’m glad to see others kicking-in.


  • Scott_F

Posted November 26, 2001 - 10:25 PM


Good info from John about the damaged mid valve shims. I have personally mangled the face shim in my mid valves. I believe they get bent up from too much pressure and flow rate. A stiffer mid valve stack or thicker face shim or greater float height should prevent this.

  • holeshot

Posted November 26, 2001 - 10:47 PM


Originally posted by Scott F:
I have personally mangled the face shim in my mid valves.

I'm not surprised, since I've actually seen you jump 100+ foot doubles....... :)

  • DaveJ

Posted November 26, 2001 - 11:01 AM


ScottF -

I'm assuming that if you have modified the mid-valve float, or supported the factory stack, that you had to make modifications somewhere else to compensate for this?

And was your shim noticeably bent?


[ November 27, 2001: Message edited by: DaveJ ]

  • Scott_F

Posted November 27, 2001 - 08:26 AM


Dave, the shim more closely resembled a corn flake. :)

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