YZ250 suspension question

Blackjack31
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Blackjack31
It seems like one of my forks isn’t rebounding all the way back out. I can pull it up and then put very little pressure on it and it falls like half an inch or so. My other fork can’t be pulled up any farther. It seems to work good and rebounds all the way. Anybody had this issue before?
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Paul_Pitzonka
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Paul_Pitzonka 12/10/2019 6:00pm
12/10/2019 6:00pm
Depends on the year... if it’s a twin chamber kyb you have a problem in the inner chamber... my bets a broken free piston or blown cartridge seal.
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m21racing
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m21racing 12/10/2019 6:18pm
12/10/2019 6:18pm
Broken spring sounds like issue to me. I've see both main, and pressure springs break in those.
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Blackjack31
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Blackjack31 12/11/2019 9:04am
12/11/2019 9:04am
Paul_Pitzonka wrote:
Depends on the year... if it’s a twin chamber kyb you have a problem in the inner chamber... my bets a broken free piston or blown...
Depends on the year... if it’s a twin chamber kyb you have a problem in the inner chamber... my bets a broken free piston or blown cartridge seal.
I just had them apart. Whenever I had them apart I was looking at the base valve assembly and the free piston had some play up and down. Very very small but I could see it being the issue. It was not cracked, holes have been drilled through it to. The nuts were tight on the base valve assembly. I didn’t take the assembly apart though. I also changed the seals that are on the base valve assembly.

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slipdog
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slipdog 12/13/2019 9:30am
12/13/2019 9:30am
Blackjack31 wrote:
Bump
The free piston is not the issue, that play is normal.

I assume you mean forks off the bike and one is "topped out" and one will sag down about 1/2", is that correct?

You can have no valving or oil in the damper and it will still come all the way back because of the installed length of the spring w/preload. If you don't have a broken main spring then I would measure the length of the springs(YZ250 should be 454mm) and check the preload(should be around 6mm +/- with 454mm spring) to verify.
Blackjack31
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Blackjack31 12/13/2019 10:21am
12/13/2019 10:21am
slipdog wrote:
The free piston is not the issue, that play is normal. I assume you mean forks off the bike and one is "topped out" and one...
The free piston is not the issue, that play is normal.

I assume you mean forks off the bike and one is "topped out" and one will sag down about 1/2", is that correct?

You can have no valving or oil in the damper and it will still come all the way back because of the installed length of the spring w/preload. If you don't have a broken main spring then I would measure the length of the springs(YZ250 should be 454mm) and check the preload(should be around 6mm +/- with 454mm spring) to verify.
I will check that. I have had this issue for a while. I then sent my suspension off to get revalved and for some reason didn’t check it. Now I decided to change the fork oil. Well I checked the spring length on one and it was good. And the other I did not cause I just figured it was good. I’m going to ride it like it is until the beginning of the race season. Then I will go back in and see what the issue is. Is there anything else it could be? Right now I have possible free piston issues and spring out of spec. And you are talking about the main for spring right? Not the little spring inside of the base valve. I didn’t take the valving apart. Just the whole assembly out to change the inner fork oil.
adam8781
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adam8781 12/13/2019 4:30pm
12/13/2019 4:30pm
if you cracked a free piston (very common) the fork will sit lower than it should and can create a suction to the shaft depending how it broke
cwtoyota
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cwtoyota 12/13/2019 5:45pm
12/13/2019 5:45pm
Paul_Pitzonka wrote:
Depends on the year... if it’s a twin chamber kyb you have a problem in the inner chamber... my bets a broken free piston or blown...
Depends on the year... if it’s a twin chamber kyb you have a problem in the inner chamber... my bets a broken free piston or blown cartridge seal.
Blackjack31 wrote:
I just had them apart. Whenever I had them apart I was looking at the base valve assembly and the free piston had some play up...
I just had them apart. Whenever I had them apart I was looking at the base valve assembly and the free piston had some play up and down. Very very small but I could see it being the issue. It was not cracked, holes have been drilled through it to. The nuts were tight on the base valve assembly. I didn’t take the assembly apart though. I also changed the seals that are on the base valve assembly.
Yamaha SSS KYB forks have some "float" in the free piston. You'll have a gap, or free play like you described.
It's about 3/8 of an inch (by eye) on a stock fork. I haven't measured them, but I would guess it's meant to be 8mm.

Did you try bleeding the air out of the forks before disassembly?
Did you measure the free length of your main springs?
Did you inspect the spring seats and spring-seat clips on the outside of the cartridges?

If your fork leg burped some air or oil out of the seal or bleeder, it could have had a vacuum inside which was fighting to hold it down shorter. (hence bleeding before disassembly)

Your issue sounded like a cartridge oil volume or free piston issue as others have said, but just offering a couple other things to check.
Blackjack31
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Blackjack31 12/13/2019 8:50pm
12/13/2019 8:50pm
adam8781 wrote:
if you cracked a free piston (very common) the fork will sit lower than it should and can create a suction to the shaft depending how...
if you cracked a free piston (very common) the fork will sit lower than it should and can create a suction to the shaft depending how it broke
I looked over the free piston and seen know cracks but next time I tear them down I will look.
Blackjack31
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Blackjack31 12/13/2019 9:03pm
12/13/2019 9:03pm
cwtoyota wrote:
Yamaha SSS KYB forks have some "float" in the free piston. You'll have a gap, or free play like you described. It's about 3/8 of an...
Yamaha SSS KYB forks have some "float" in the free piston. You'll have a gap, or free play like you described.
It's about 3/8 of an inch (by eye) on a stock fork. I haven't measured them, but I would guess it's meant to be 8mm.

Did you try bleeding the air out of the forks before disassembly?
Did you measure the free length of your main springs?
Did you inspect the spring seats and spring-seat clips on the outside of the cartridges?

If your fork leg burped some air or oil out of the seal or bleeder, it could have had a vacuum inside which was fighting to hold it down shorter. (hence bleeding before disassembly)

Your issue sounded like a cartridge oil volume or free piston issue as others have said, but just offering a couple other things to check.
I measured the spring length on one of the forks but did not on the other. I’m not 100% sure what clips you are describing. I will have to look into it and find that out. I’m just stunned that I took it to a suspension guy and it’s still not right. So one reason I took it to him was because I could pull up on my bars and it would stick up, then put slight pressure and they were drop a little. Well he got done with them and they felt softer and I didn’t even think to check the sag issue. Well I decided to take them apart and change the oil. Whenever I had the cartridges pulled apart before complete disassembly (I forgot to mention this), I noticed one dampening rod would come all the way out on its own and the other stuck at the very end and I had to pull it out the rest of the way. (Probably 1-2mm, very little but I could tell) Well in my head I just thought it could be old oil, air in the fork, whatever. Well I changed the oil and put it back together and it still wouldn’t come out all the way. So it was actually not completely working when it was just the cartridge, no main spring. So it has to be something in the base valve assembly or something is causing to much friction for the dampening rod to come out. It might could be the free piston but I swear I checked it really well because I’ve heard of the pistons cracking. Next time I take it apart I guess I will hit it with a magnifying glass or something. Should I take it apart now? Just sucks cause that oil for the inner cartridge is far from cheap. I mean I rode it with it for like 6 months like that. Just got terrible arm pump because it felt super stiff. Sorry for the long response but I just hate to take it to a shop and they charge me $200 labor for something I can do at my house.
ristaone
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ristaone 12/19/2019 5:19pm
12/19/2019 5:19pm
Hi there I have a 2007 yz250f and my rear shock is wrecked the bike is bucking me upwards over the smallest jumps so I was going to replace the rear shock but my mate has a rear shock from a 2015 yz450 but I'm not sure if it will fit
Can anyone tell me if the 2015 uz450 rear shock will fit on my 2007 yz250f

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