1989 Yamaha YZ125 info.....

I just rebuilt one, wished I started off with something that looks like yours. I haven't ridden it yet. Plastics, I got from UFO. Fork springs, I haven't found any due to 89-90 having 41mm forks. Supposedly the 91 forks(43mm and you can get springs for these) and triple clamps bolt right on and you can use your wheel, axle, and caliper. An upgrade would be to install a 91 or 92 outer clutch cover. The 89-90 are magnesium and flex making the clutch drag. 91-92 are aluminum and are stiffer. Speed and Sport Yamaha was a great help in locating those hard to find missing parts. Their website might not have a part listed, contact them, they might have it or they will try and find it. Like any 29Y.O. bike, some parts are NLA. It just takes a lot of searching to find what you need.

Currently building the same bike! I concurre with everything Dekon has said. My bike is a roach so I have lots of research in on parts, so fire away if you have specific questions. One problem area on these bikes is the rear caliper. The caliper pins get badly corroded and are NLA through Yamaha. I’ve figured out late model KX85 is the same caliper and carrier and all the parts are available there.

chuckie108 wrote:

Currently building the same bike! I concurre with everything Dekon has said. My bike is a roach so I have lots of research in on parts, so fire away if you have specific questions. One problem area on these bikes is the rear caliper. The caliper pins get badly corroded and are NLA through Yamaha. I’ve figured out late model KX85 is the same caliper and carrier and all the parts are available there.

Not to hijack, but Chuckie, what years KX85 caliper works? The only place I found that had a caliper rebuild kit was from the U.K. Cost almost as much to ship it as the kit was.

Another area with a limited parts availability is the clutch basket and inner hub. Was I knew if another year was able to swap in its place.

Also I've been looking and can not find the rear brake pedal spring, part 90506-14410-00. Right now I using a later model one, but it lifts the pedal a little to high for my liking. If you come across something that has a closer length to the original, please let me know.

Dekon wrote:

Not to hijack, but Chuckie, what years KX85 caliper works? The only place I found that had a caliper rebuild kit was from the U.K. Cost almost as much to ship it as the kit was.

Another area with a limited parts availability is the clutch basket and inner hub. Was I knew if another year was able to swap in its place.

Also I've been looking and can not find the rear brake pedal spring, part 90506-14410-00. Right now I using a later model one, but it lifts the pedal a little to high for my liking. If you come across something that has a closer length to the original, please let me know.

2010 to current KX85, you can buy the carrier assembly, or you can buy the seals, pins, etc. seperately.

The clutch I haven't totally solved- yet. There are zero 89-90 hubs out there that I have seen. 89-90 is a six spring, 7 plate clutch. 91-92 is 5 Spring, 7 plate, and 93-05 is 5 spring 8 plate. If you want to run a 93 and newer combo, You need a 93 RT side center case. Im still figuring how to solve this. I know the inner hubs will work on our bikes(as far as inner and outer splines), but we will need a 5 spring basket to go with it, and that part I havent solved yet. I'll keep you posted. I'm sure there is a way to make newer components work, but I just havent gotten there yet. I'll keep you posted.

Brake pedal spring- I have no idea. There's gotte be a newer version out there that will work. Or maybe even look at Mcmaster-car for something that is workable.

I raced that bike from 89-90, and ended up with a 90 model for about a year a while back.

The bike is surprisingly good for its age. 88-89 was really when the shape of modern bikes came into being with low boy layouts, USD forks and disc brakes. 89 had a punchy mid range motor and a comfortable layout. I raced and practiced on that bike, rode multiple times a week, and with regular maintenance (air filter, trans oil, top ends) the only thing that ever failed was the CDI box. I never had any issues with the clutch, and was a 125 B rider by the time I sold it.

I really hope to find another 89 at some point! Enjoy!!!

Yamaha also likes to share parts from model to model, so you will probably find a lot of things that cross over anywhere from 86 or 87 to the early 90s in that engine.

This is the type of info I was looking for, thanks everyone! Please post any additional information on this bike you run across during your builds. Thanks again!

Just to provide more specifics to the info already in this thread. I received the caliper carrier and seals this week. Here is a pic of the old parts next to the new parts along with the part number for each. That way if any '89 YZ owners stumble across this thread trying to find a solution to the sticky caliper which I have seen a few of on '89 YZ's.

chuckie108 wrote:

Just to provide more specifics to the info already in this thread. I received the caliper carrier and seals this week. Here is a pic of the old parts next to the new parts along with the part number for each. That way if any '89 YZ owners stumble across this thread trying to find a solution to the sticky caliper which I have seen a few of on '89 YZ's.

Thanks Chuckie108 that's is super helpful.
Much appreciated.

Found these on ebay, might help someone.

https://www.ebay.com.au/itm/Yamaha-89-90-YZ125-YZ250-Fork-Springs-4-42kg-mm-May-fit-KX125-250-RM125-250/252494506464?hash=item3ac9d869e0:g:gAgAAOSw65FXqkno

nice detective work there Chuck. i think that caliper mount might fit the 250 as well.

Hey i have a 1990 yz125 i got for a $170 bucks but its a project bike it needs a new inner clutch hub and i cant find any at all do anyone no where i probably find one at or what other year inner clutch hub i could get to fix my problem please

I ran back in 89 a 1988 model fmf pipe and shorty style muffler. The 89 fmf wasn't good. The 88 model pipe made a huge difference and the shape was more like Bradshaws. If i remember the guy who did the motor had to weld some brackets different for it to fit though.

DC Plastics has tons of vintage MX stuff.

Just finished this 1990, the clutch was hell! I ordered a 91 cover for the clutch that is aluminum and used OEM plates and springs only. The Arm is key to have in the adjustment window on the cases. Make sure that it stays within the adjustment window ! I cant stress this enough!