Fork update

18 replies to this topic
  • DaveJ

Posted March 23, 2001 - 03:38 PM


For those that monitored the fork re-build thread between Scott F. and myself, you'll recall my saga of repeatedly losing compression in my 426 forks.

My purpose on following this up is not to stir up any concerns, but more to just let you know what to look for and possibly what to do about it.

At intervals varying from 20 to 40 hours of riding, I lost compression in my forks. Three times. The bikes handles poorly when this occurs, but it's possible that it could go un-noticed for some time since handling performance slowly degrades.

The best way to test is to simply lock the front brake and bounce up against the forks. If they easily compress with little or no hydraulic resistance (it will feel like you're bouncing up and down on the spring), then you may have the same problem that I encountered.

Originally, I assumed this was an issue with the seal heads. What I have now learned is that it appears to be a mid-valve failure, but I'm still not sure what is causing it.

It appears that the mid-valve slowly fads in performance over time which allows too much oil to flow past, which in turns forces te oil out through the seal heads, and of course produces little or no resistance.

I spent a significant amount of time testing the mid-valve and it's partnering check-valve in a series of configurations with no results concluding that no physical part had failed, (such as shim fatigue). The type and quality of the oil was eliminated as well.

However, when I rebuilt the mid-valve, using all the originals parts, everything worked. What is a mid-valve rebuild? Complete disassembly, smoothing of the piston surface, cleaning, and then torque to spec. All the same shims, springs and plates were used.

I eliminated the possibilities of the other fork components failing, such as compression valve assembly or seal head, since I was able to isolate just the mid-vale and rebound stack during my testing.

If you find that this is an issue with your bike, please respond. I’d be curious to know if there is perhaps something particular to my forks or if this is a common concern with the use of a mid-valve in the late model KYBs.


  • Scott_F

Posted March 23, 2001 - 04:55 PM


Dave, did you flip over the mid-valve shims? :)

  • Vester

Posted March 23, 2001 - 05:52 PM


Hey, you need to send Jeremy at an email. He can explain to you whats happening with the mid valve when it gets over stressed. They hashed this out at the Suspension forum on DRN last year. I cant exactly remember what the culprit was but the fix was simple. I always though it was an over flexed shim.

96 RM250

  • Guest_Guest_*

Posted March 23, 2001 - 07:55 PM


The problem is usually the midvalve. Although, the cylinder valve can get full of sludge and leak which wood cause a similar problem.
I wouldnt re-use the shims if in fact they are warped. New ones are only a couple of dollars.
One of the problems is removing the cylinder valve, because the cylinder is peened around the cylinder valve. If you do not relieve the peens properly you stand a very good chance of galling the threads upon removal.
Use a 6mm end mill and go about 1.35mm deep into the peen.
Another good idea, but one that is a little tedious is taking apart the cylinder valve apart for cleaning. This is a pretty involved process that I wouldnt reccomend to the faint of heart. Just try a clean it the best you can after you remove it from the piston rod. And blow some compressed through it.
Unfortunately, Kayaba assembles thier YZ forks as if they never want someone working on them.
Take Care, John

  • SUnruh

Posted March 24, 2001 - 06:23 AM


actually, it was Shocknut (Jeff Howe) that came up with the fix. at the time he was an affilate of Jeremy's (MX-Tech) and so passed the info along. not anymore. if you want the info straight from the guy that came up with the fix (and several others, btw) is the place you wanna go.

  • Vester

Posted March 24, 2001 - 01:14 PM


You're right,my bad, It was Jeff. I just remember them discussing this in full detail. Pretty interesting stuff though. I wish I could've saved that information.
How come Jeff is no longer an MX-tech associate?


  • DaveJ

Posted March 24, 2001 - 05:49 PM


So it's a little disappointing to find that this is a more common of a problem than I thought.

When I inspected the mid-vale shims they looked fine. I'll have to assume that through the process of cleaning and rotating them among the stack, that I was able to re-gain my compression...for now. (Scott - does that answer your question?).

So what's the real fix here? I would have to assume that it's either replace the mid-valve shims on a regular basis, or remove the mid-valve stack.

But then I would also have to assume that if you remove the mid-valve stack that a different rebound piston would have to be installed - or other mods have to be made elsewhere, of which is questionable.

I had tried this once using Race-Tech's approach to removing the mid-valve. Needless to say, it ruined the handling performance of the forks.

When I first installed the works style bottoming cones and ultra adjusters, together with a fairly fresh mid-valve, the forks were perfect. I'd be afraid to deviate from this configuration even if it means replacing the stack every 40 hours or so.

And did I miss something or do you guys think that I should have been able to tell worn shims from good ones?

The other concern is what everyone else on this forum is oding about this problem? What about all the other KYB forks on the other YZs?

I don't think most suspension shops are aware of this, or would consider breaking into the compression tube as part of a regular service.


  • Scott_F

Posted March 25, 2001 - 06:34 PM


Dave, why are you still unregistered? Take off your shoes and stay a while.

I suppose in theory if the tired shims are flipped over they may be stiffer. If you are sure that you have a recurring problem with the mid-valve shims, then, yes, you should periodically change them. You can also tune the stack just like any other stack, by changing quantity, size and thickness.

It might be tough to tell a fatigued shim, without some sort of scientific test, unless it has physical signs of damage, like I have seen in some mid-valve shims.

As for "removing" the mid valve, you have to replace it with a check valve, similar to the one on the top of the compression piston. This is usually done for enduro and other super plush off road requirements. You don't replace the piston.

  • DaveJ

Posted March 25, 2001 - 08:10 PM



Here's what spinning around in my mind at the moment.

First, it looks to me that the mid-valve is simple a stack of shims that sits in front of a check valve. These shims add additional resistance to the oil flow when the piston is traveling downward (a compression cycle). I'm just stating the obvious.

Remove these shims and you have a basic check valve already in place. No need to make one.

However, when I removed these shims, compression resistance was way too low. Too low to even offer enough resistance for someone wanting a softer ride.

So I assumed that perhaps KYB designed the piston to operate at a flow rate with consideration to a mid-valve being in place.

So if one were to remove the mid-vale shims, something else on the fork would have to be modified to compensate for the lost resistance.

Why the argument to remove the mid-valve? I would have to assume that the progressive feel that shim stacks offer might be too much for some riders.

Race-Tech has this thing of removing the mid-valve, replacing the check valve spring with a stiffer one, then installing one of their custom restrictor plates under the seal head. The result is poor handling, but it perhaps supports the theory that flow rate has been considered into the piston design for a mid-valve to be in place.

I'm still waiting to hear back from these guys at TrakControl on what their take is on the issue.

So anyways, I will have to assume that in the process of flipping over and moving about the mid-valves shims that this is what fixed my problem.

However, so far history has told me that within the next few months, I'll be back into my forks dealing with that mid-valve again.

By the way, did you have the same problem?

Did you remove the mid-valve? What other mods have you done other than the bottoming cones and compression stack changes?


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  • Boit

Posted March 26, 2001 - 12:17 AM


For those of us who want to service our forks....and maybe do a little internal shim adjustment.....What should we look for? From previous threads, I see that we should watch out for removal of the seal head. Since KYB deems it necessary to punch in thread-locks, how should we proceed? I don't want to damage my KLX forks.....nor do I want to cause irrevocable damage to my MX-Tech forks.

  • DaveJ

Posted March 26, 2001 - 11:03 AM



My criticism of these forks are too much high compression and not enough consideration to a proper bottoming system.

Compression rates are really the art to proper handling. Too much and you skip all over the place. Not enough and your ride is unable to react.

The rule of "plush but firm" remains the best description.

Bottoming resistance on these forks is a combination of a bumper system and a small hydraulic effect via a piston-ring that enters the top of the seal head.

However, high compression is really the issue, and I think this has yet to be resolved from KYB or Yamaha. Based on what I've read about the newer models, mainly the 250F, they have made some changes to the bottoming concern by increasing the hydraulic effect.

So what is one to do?

Based on my experience, I was not able to fully resolve the high compression issue via the valve stack at the bottom of the fork - nor were two of the suspension shops that I worked closely with. Regardless of single or two-stage stacks, it seems that you cannot buffer enough high-compression without sacrificing low compression. In other words, it gets mushy.

From there, I tried modifications to the mid-valve, which only opened the can-of-worms further. It seems the mid-valve has to stay in place. For now.

With no where else to go, I took some bold steps based on recommendations from Scott F. and a few others in the biz. It also helps to walk around the pits or take a close look at some of the factory bikes. However, this involves some serious modifications to the forks.

My end result was the installation of what they call "bottoming cones", a complete replacement of the factory seal heads and removal of the bumper system. A lot of cut and grind. And even then, there are additional issues that arise beyond the installation instructions from the manufactures. Tricks of the trade I guess.

In addition to this, I removed the factory compression assembly, and installed what are called "ultra-adjusters", which allow external adjusters for both low and high speed. These still use the Yamaha valve/piston, (which, by the way, is far better than any aftermarket unit you can buy), and also uses a fairly traditional shim stack.

The bottom line, beyond my most recent concerns, is a fully and easily adjustable fork that rides very smooth, sticks very well, yet is firm enough to maintain full control at all speeds. It's truly amazing what a difference it makes. It's also amazing how much of a secret it remains.

My only concern, as you may have read, is that the mid-valve seems to weaken, somehow, over a period of 20 to 40 hours of riding. Remove this valve with no other design consideration and you seem to lose too much compression for the bike to handle.

I will pull it, but only when I can find someone that can really explain how to compensate for its purpose. Till then it stays, and I will continue to rebuild the forks (umm…mid-valve) as need be based on the benefit of the ride I’m getting.

In other words, you have to really get into it, or stay out. Or get different forks. If you have the cash, and walk in with enough wisdom, I’m sure you could find the right shop to go at for you.

Let me know if that helps.


  • motojunkie

Posted March 26, 2001 - 07:33 PM


I've heard that Factory Connection ( 1-800-221-7560) has a fix for the mid valve problem - you might want to give them a call. Every time I've called they were more than happy to share information. Good luck.

  • DaveJ

Posted March 27, 2001 - 10:44 PM



Thanks for the tip.

I just got off the phone with Tim at Factory Connection. Seems to be one of the few that really understands these forks.

He confirmed the issue with the mid-valve, and modifications to its collar to help alleviate stress on the shims. I had tried this as well, but with not as much precision as he spoke of. This seems to be the most logical approach I have heard yet.

This whole issue has really taught me that very few suspension shops really know what's going on. In addition to this, really good shops, such as this, seem to be real particular about designing and setting up forks for each rider.

In other words, you have to expect to have a long talk with these guys about your needs and riding style before they "service" your forks. A good shop should really understand the dramatic differences between a fast trail rider and a Supercross wannabe, (like me).

So two thumbs up on Factory Connection.

Hope this helps.


  • SUnruh

Posted March 27, 2001 - 12:03 PM


you need to talk with jeff if you want to know why.

ah, you have seen the light. jeff set me up with the cones. if you bottom them, you were in for a handful without them for sure! :) they are awesome, i won't own a bike without them ever again. you also ought to talk with jeff. he has more than one way to skin this mid-valve problem.

  • DaveJ

Posted March 27, 2001 - 02:17 PM



I wrote Jeff, but his reply was more focused on making money than talking about the issue.

And yes, the cones are the way to go. They can be a real life saver as well if you case a jump.


  • motojunkie

Posted March 27, 2001 - 10:24 PM



I'm glad Factory Connection helped out. They re-valved my CR250 ( I'm a little heavier than the target audience ), and did an awesome job - I didn't even have to set up race sag, it was spot on! I'd recommend them to anyone.

  • Scott_F

Posted March 28, 2001 - 10:59 PM


So what's the spec Dave?

Originally posted by DaveJ:
He confirmed the issue with the mid-valve, and modifications to its collar to help alleviate stress on the shims. I had tried this as well, but with not as much precision as he spoke of.

  • DaveJ

Posted March 28, 2001 - 02:42 PM



He was a very helpful guy, but he didn't seem like he was ready to give that information away.

During my testing, I had tried a shorter collar, the one from the bottom assembly, and it produced way too much compression. In fact, it may have placed the check valve plate right on top of the mid-valve, (can’t remember…so of course I would get this result…dah!). I guess I was just testing if it was the mid-valve….?

So….I would think the magic number would be as close as you could get without an increase in compression. Right? Or slight.

I have a spare set of mid-valve assemblies in my garage that I can play with. I'll come up with a number and post it.


  • Scott_F

Posted March 29, 2001 - 11:36 AM


Dave, post the dimensions on the collar. It would be easy to make some shorter ones, like in increments of .5mm. Or, IIRC, you could just shim the back side of the mid-valve to get the same effect.

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