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Bad KYB Shock Service?

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Dec 6, 2000
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#1
My friends shock free-sags properly before riding, but after riding, the shock "sticks" somewhat and the bike tends to stay where it's pushed. He just had the shock serviced by a local shop and they can't find any problems with it sticking, even after heating it up with hot water. Has anyone else seen this problem? It's a '00 YZ250 KYB. What could make it bind with heat? The shaft is NOT bent.

Mark
 

marcusgunby

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#2
ive seen one do this after bieng serviced by PA but im sorry i never found out why-maybe the seal on the piston assembly is swelling more than the body so causing stiction but why i dont know.
 

Jeremy Wilkey

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#4
Mark,
I'd go with a bad PA piston, or linkage bind.. Linkage bind tends to be all the time however..
Regards,
Jer
 
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#5
We checked for linkage binding and that's not it. The seal swelling may be more on target. The shock body and piston are both aluminium and should expand at the same rate, so I can't see the piston binding. I was thinking maybe a seal/bushing was getting soft and "wedging" or misforming into a place it didn't belong. The weird part is the stiction always disappears when the shock cools off. It's been disassembled and inspected once already, with no apparent problems. It's going back again, but I have doubts as to the outcome. This problem developed immediately after the first service, so I'm convinced the tuner introduced this problem in some way. Maybe a seal was installed incorrectly or was damaged when the shock was assembled? :think

Mark
 
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#6
Could be the rebound rod inside the shaft sticking in the orifice?? I've had a few in here with this problem in the past.
 
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#7
That sounds plausible. I'll check it out and post the outcome when the problem is finally fixed.

Thanks to Everyone!

Mark
 

Jeremy Wilkey

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#8
Mark Shockut...

I've noticed that is a problem with the shocks more when they are cold or low on gas.. Should be intresting to find out! I had a guy bring his bike in with the rear colasped and stuck form that problem...
Regards.
Jer
 
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#9
Jer,


I can't tell you what it is for sure that caused it. 2 of the shocks were from KX's but none of them had been touched by PA. Is it possible that air pockets from cavatation would hold that needle up tighter. I've always wondered, though I never checked the pressure and never found anything out of whack internally. it's been awhile since I've seen it, better than a year now, but I'll do a better job of looking into it on the next one I get like that. It bothered the heck out of me why I couldn't diagnose it but I still think it had something to do with air in there.

But, heres something weird... I had a shock from a 01 CR125 few weeks back, brand new, never ridden on yet that had a measureable pressure of 120psi in it, factory charge!! I wonder what that would have felt like after a long moto??
 
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#10
Hey Guys I have had several KYB shocks do this. Mostly Kaw but a couple others also including a brand new 01YZ250. I have seen this in the last 12 months or so and this is what I have found so far.

I have replaced everything (even diff. oils) inside several shocks and found that the bodys have been the main problem. I was told by a couple other suspension builders that there has been some bad KYB bodies in the last couple years but not bad enough for any kind of recall.

I had a reputable machinest look at a couple of bodys and he determined that they were made egg shaped and that what was happening was that the body would swell and actually get smaller on the inside. He actually built a jig and pressed the body (from the outside) while keeping a dial guage on the inside and pressed the body back round. That shock is still working after trying a new OEM piston, and shaft assy first which didn't work.

The rest of the shocks I've just replaced the body and they have worked fine. The only thing I can't figure out is why sometimes its a brand new shock that hasn't been apart and sometimes its a 6 month old shock that has been working fine then all of a sudden 1 day it locks up as soon as it gets hot.


doc
 
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#11
Thanks for all the help guys. As promised, I said I would follow up when the problem was resolved. The suspension rebuilder measured the shock cylinder carefully and found it to be good. He then removed an o-ring that sits underneath (covered) by the bushing in the shock piston. Stiction dissapeared! I'm not sure what function this o-ring had, but the shock has performed fine ever since this change was made. :cool;

Mark
 
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#13
It was the stock piston. The problem may have existed before the first service and just went un-noticed. I'm not familiar with the design of the piston. I've seen the teflon bushings on forks I've worked on in the past, but I have not seen o-rings used underneath, above, or below those bushings. The way it was described to me, the o-ring was placed underneath the bushing.

Mark
 
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#14
YIKES! The stock piston uses 2 0rings underneath the piston band on the KYB design. Removing one of them is a scary deal in my opinion.
 

marcusgunby

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#15
I did a friends 2001 RM 125(Showa) with only 10hrs riding on it and the seal head assembly was almost too tight to go into the body(sounded like sandpaper while stroking the shaft).The seal diameter was 50.3mm and the body was 50mm.I removed the o rings behind the teflon band and the diameter reduced to 50.1mm.It then ran smoothly in the body.He rode it and was happy with the performance.I will inspect it mid season and may end up refitting the o ring to take up any wear.