Changing oil on 05 YZ125 Kayaba forks

[MXBrian]

I've got about 5 race weekends on a new 05 YZ125 so I figure it's time to change out the fork oil. I'm fairly inexperienced with the new twin chamber forks and was hoping to have a few questions answered:

1. The Yamaha manual specifies "S1" fork oil. I'm assuming this is a Yamaha designation. Anybody know what weight S1 is equivalent to, and does any brand/model of fork oil stand out as superior to others?

2. It appears that I'm going to have to basically completely disassemble the forks in order to drain both the damping assemblies and the fork tubes. Am I figuring correctly? I ask just because I want to know if I'm going to need all three tools: fork cap bolt ring wrench, fork cab bolt wrench, and fork seal driver.

3. I've seen mention of shimming-up the fork springs and compression damping adjuster (cartridge) springs due to a couple mm of free play (no preload). Is there a good reason to do this and, if so, what size and type of washers do I need?

4. The manual only specifies fork oil volumes, not levels. It looks like the damping assembly just gets filled then you let the excess flow out of an overflow hole while compressing the assembly fully. Am I correct me on this?

5. As far as the actual fork oil level proper does someone know what the stock height is? Can I even use a fork oil level gauge (like Motion Pro's) with the Kayaba twin chamber design? It looks like maybe the spring/chamber assembly (which I think has to be installed before filling) would be in the way if I tried to use the gauge, and I wouldn't know how much of i the assembly to have in the fork while measuring, which would affect measured oil level. Is going by fluid volume, instead of fork oil height, pretty much the only way to do this properly?

6. Anybody have a recommendation on how often to change the fork oil on this fork? The manual says "every 5 races" which isn't too helpful...don't know if they're talking race *days* (and how many motos a day) or actually every 5 motos. Not like I'm doing 30 minute motos, anyways. I race every weekend, 6 4-lap motos a day, and generally practice once or twice a week. Any suggestions?

I really appreciate any help you guys would offer up! Any other tips anybody could offer up to me in doing this maintenance for my first time would also be appreciated.

 

[osheen]

1. I use Maxima 5wt.(85/150) I was a little thrown off by the "S1" spec too.
2. Yes, you have to take the fork completely apart. You really don't need to separate the inner & outer tubes though unless you are changing seals. You will need the special wrenches. The wrench areas are 8 sided, not 6 like a normal wrench so you pretty much have to buy the tools. It's hard to improvise anything else.
3. I know Race Tech and Pro Circus recommend adding the spacers. I made my own on a lathe at about 7mm in length. I put them on top of the spring like the manual shows. Race Tech's website says to put them on the bottom. (That's where to put them on the old stlye forks but with the new design they need to go on top.)
4. You are correct. They specify an amount but you just need to be sure that the oil level is above the step when pushing the base valve into cartridge. I think the more oil the better (because it will help to bleed more air out) but then it becomes real hard to compress the base valve into cartridge. After you get it together hold it upright and stroke it a few times just a couple inches. Next lay it on it's side with one hole facing up and finger over bottom hole, completely bottom out the damping rod and it will bleed the air out. When damping rod is fully extended it won't really want to top out. It kinda wants to pull back in a little and feels like there is a bit of air still in it if you pull on it. (which there probably is). That's as good as it gets, so I've been told.
5. Yes you have to use a volume measurement. There really isn't any way to measure height because the spring and cartridge get in the way. Also the oil level is really low because there isn't much in there compared to old fork. The outer oil only lubes the bushings and seals and takes up air space. Be sure that the fork is completely drained the best you can before filling again. Unless the forks are completely disassembled and reassembled dry, there will still be a few cc of oil trapped inside. They call for something like 245cc of oil. I wouldn't doubt that there is still 20cc of oil still in there unless you take it apart. I wouldn't worry about it too much, just be consistent in how you do it and you can alter the amount to suit your needs.
6. I don't have a whole lot of time on mine but I've been told the outer oil gets dirty a lot sooner than the inner oil. Myself, I plan on changing it about every 6-8 rides. I know some guys that go all year without ever changing it. I'd just keep an eye on it's condition over time and change it when your concious tells you to.

I hope this all helps. Get yourself the tools and just go for it. Follow the manual. It all becomes a lot more clear when the parts are in your hands. One other thing, be prepared to make a big mess. You'll have oil all over the place. Way more messy than the old forks.

 

[MXBrian]

Thanks

Thanks, Osheen, for the great feedback. It's good to see that after pouring over the manual I wasn't too far out to lunch. I figured out that I don't need to separate the stanchions from the sliders (and thus would need tto re-seat the seals) after I posted, so that was good to read. Basically, you saved me from buying a tool that I don't necessarily need at this point (the seal driver) and another that would be useless to me (fork level gauge). That's like $100 you saved me!!

Great bike, though, huh? I love it. Just put an FMF SST pipe and Powercore silencer on before running down a few states and doing an arenacross and I was really pleased with the power and response gains I felt I got. Also have a VForce-3, but haven't put it on yet.

Have you messed with your jetting at all? I fouled a couple plugs when the bike was cold a couple times this fall duirng a couple real hot days, then did the same exact thing down at the race when it was in the 20's! I'm thinking of going with a couple articles I read that suggested moving down one or two sizes on the pilot and then raising the needle 1/2 slot.

I've also been running pretty low octane fuel (92 is the highest I can get around here, the manual says 95 or higher) which probably isn't helping me out. I just realized that I can get 100 octane AV gas at the airport across the street from me, though, so I'm hoping that'll help a little. I haven't heard any pinging, though, and I'm not sure too-low octane fuel would cause plug fouling anyway.

Well, thanks a lot for the thorough reply -- helped a lot!

Brian

 

[roadrash1]

Been hearing a lot of good things about that YZ 125. That is one light bike!
You may want to rethink your idea of Avgas though. It really is made for low RPM air-cooled motors that run a single power setting. Consider an octane booster to the premium pump gas, or a real race fuel. I can tell you the dyno would show less power with Avgas than even the pump gas without an octane booster.

 

[MXBrian]

Thanks

Ok, I'll take that into consideration.

Thanks,

Brian