YZ125 1979 Race build

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9/9/2013 2:22 PM

I picked up a YZ125 a couple months ago and started the cleaning and inspecting process. I also started collecting parts and researching forums for tips and ideas.

Here's what I brought home for $400
[LINK TO IMAGE]

Among all the interesting things I found that PO had done to keep this thing running was an addition that may or may not have been a mistake. See the black dot on the tank? That's a .22 slug. Yep, it's 1/2 way embedded in the tank and the PO thought it was JB weld from the owner before him.

So-Far I've replaced all bearings and most seals. levers, grips, decals, airbox, side panel, Stator, Rotor, coil, and a ton of little detail things that were very wrong.

I'm almost satisfied witht he bike now. She still heeds the bars straightened and the DG silencer mounted. Sounds like a machine gun firing through a tin can...

One thing I may need help with is suspension: I can bottom the rear by jumping on / leaning back. And it seems to hit a hard stop sooner than I'd expext it to bottom. Has anyone here rebuilt these? I've got a 1979 IT250 for spares, so maybe I can build a better boinger from what I've got?[LINK TO IMAGE]

Any help / suggestions and hot setups from back in the day would be great!

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9/9/2013 2:25 PM

This is what she looks like now.
[LINK TO IMAGE]

Oh, and the IT250 has taller suspension and a reservoir on the shock. Also has solid handlebars with no crossbar! Should I swap suspension????

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9/10/2013 7:52 AM

SUSPENSION: The 125 forks are 36mm and the 250 forks are 38mm. I measured the lengths and the 250 forks are an inch longer as well. I'm thinking about swapping suspensions, but I'm not sure if it would make the bike better for me or not.

I'm not a fast rider anymore, middle 40-C class racing my 1987 YZ490 against modern bikes. And I weigh 190 instead of the 145 that the bike was built to carry around.

OK, I think I just convinced myself on the suspension swap. Stay tuned. The 250 forks are good but need rebuilt. The shock is a big mystery: Supposedly it won't fit in the frame without some modifications to the frame. It needs a full rebuild as well.

MOTOR: The bike runs well, but seems to have low compression. I'm used to kicking over a 490, so I'm a little biased. I popped the pipe off and took a look into the EXH port. There's a brandy new piston in there! The PO told me it had a new top end before the ignition died, but that's what they all say.... So, it's either not even broken in or it was done wrong, or maybe the loose sparkplug has something to do with it... Like I said, everything was wrong with this one, but at least it was almost complete. The ports look bone stock too, which is great news. I can't wait to get in there and bring the porting up to modern specs.

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9/10/2013 12:11 PM

Sweet! Bike looks great!

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04 KX125
94 KX250SM
75 Z1-900

9/10/2013 12:25 PM

Thanks Kidd, It would be cool to moto against you and your RM125. I think the RM is a better bike, for the time being heheh... tongue

I think the RM has 10-11 inches of suspension travel. My YZ has 9 I think, and I can bottom it with my arse. pinch

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9/10/2013 1:11 PM

Sounds good, last one to foul a plug or throw a crank wins!

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04 KX125
94 KX250SM
75 Z1-900

9/11/2013 7:00 AM

I recently sold my 79 YZ125 after making similar mods. I fitted a set of 38 mm yokes and forks from a 1982 YZ125, and they were a bit better than the originals, but as you have noticed, they are fair bit longer, so you end up running them way down in the yokes otherwise the bike looks like a chopper and doesn't like turning.

I think you will find the rear shock from the IT is longer than the 125 shock, and if it is you will not get it in there. I tried a slightly longer shock in mine, and it just would not work. Here is a pic of mine as it was sold with the 38 mm forks installed. You can see how far down I had them set in the yokes. This was as low as I dare take them and yes, the wheel did just touch the underside of the guard at full travel.

Photo

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9/11/2013 8:20 AM

Thanks for the tips Murph. I've already fot the stock forks up in the triples, but maybe that's because the shock is goofy. I don't think it will ever corner like an RM, but I'm going to try to tune it in best as I can.

I've heard of guys installing 250 shocks, but having to modify the frame a little at the opening since the spring is larger in diameter. This is something that I'll be investigating. The shock on my parts bike is very rough.

What pipe is that on your bike? and more importantly, did you like it? And even more importantly, did you measure it?

Nice Hannah lightening stripes haha cool cool

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9/11/2013 1:44 PM

Gotta have lightning bolts on a YZ, it's the law w00t

I think you're right about the YZ turning capabilities. It always felt a bit reluctant to turn tight, unless it was just me. blush

The pipe is a Retro Rocket from Vintage Iron. Nicely made and good fit. Had to tweak the jetting a bit, the bike ran a bit richer with this pipe. Definitely a good pipe though.

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9/12/2013 7:37 AM

I think the forks on my organ donor bike are the same as you're running Murph. So, there is one improvement that can happen.

I keep reading about the rear suspension, that it's weak and short. But also that the shock body is the same as the 250 and 400, but that it has a larger spacer in it that limits travel. I hope these shocks are rebuildable, or at least take-apart-able without too much hassle. Hopefully I can lengthen the travel and get a stronger spring to fit in the frame...

The previous owner has HD clutch springs installed and even with a new cable the pull was very heavy: unacceptable! I opted for the MOOSE RACING EZ-Clutch. It took a little work to get it right, but I do like the feel of the clutch now. It's about as easy as my YZ60. grin cool

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9/12/2013 10:54 AM

Tdub13 wrote:

This is what she looks like now.
[LINK TO IMAGE]

Oh, and the IT250 has taller suspension and a reservoir on the shock. Also has solid handlebars with no crossbar! Should I swap suspension????

Nicely Done!!

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9/12/2013 2:36 PM

Murph wrote:

I recently sold my 79 YZ125 after making similar mods. I fitted a set of 38 mm yokes and forks from a 1982 YZ125, and they were a bit better than the originals, but as you have noticed, they are fair bit longer, so you end up running them way down in the yokes otherwise the bike looks like a chopper and doesn't like turning.

I think you will find the rear shock from the IT is longer than the 125 shock, and if it is you will not get it in there. I tried a slightly longer shock in mine, and it just would not work. Here is a pic of mine as it was sold with the 38 mm forks installed. You can see how far down I had them set in the yokes. This was as low as I dare take them and yes, the wheel did just touch the underside of the guard at full travel.

Photo

Who did the pipe on your 79 ?

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9/13/2013 7:18 AM

It's a Retro Rocket from Vintage Iron.

Question: Has anyone here rebuilt the shock? The parts fiche doesn't ever show the shock exploded view for shocks. But All Balls Racing offers a seal head for this bike: P/N: 37-1116.

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9/16/2013 7:17 PM

I made a small amount of progress today: 1/2-way mounted a DG silencer to the bike. Still need to fabricate better mounting brackets and weld the inlet pipe. Like the title says, this is a race build. Expect some welding rod engineering. whistling
But hey, at least it won't sound like a chainsaw anymore...blink cool

[LINK TO IMAGE]

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9/25/2013 7:13 PM

Still haven't found the time to start on the suspension upgrades. I've looked on Youtube and googled it as well. I can't find any info on rebuilding the shock. I guess I just need to dive into it as soon as I get the chance. If anyone out there has info on rebuilding vintage Mono-shocks please let me know!

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9/26/2013 3:30 AM

Sage Suspension can rebuild the shock and it will be better than new. He will make it dampen better and get maybe another inch of travel.

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9/26/2013 7:37 AM

Thanks, I'll look him up.

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9/30/2013 9:47 AM
Edited Date/Time: 9/30/2013 9:52 AM

Just grabbed the most important part of the bike, which was missing. The Manual. Lots of good info in there...

Hopefully it has ignition timing specs, which is one of the things I need. I also reference the pictures in the manuals for wiring, cable routing and numerous other little details which have been messed up over the years of abuse. There's just so much on these old bikes that has been molested and done incorrectly. I'm amazed how some of these bikes even ran.
[LINK TO IMAGE]

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10/8/2013 4:42 PM

I took the shock off the parts bike and took a look inside. Yikes! At least they used one shim.
Photo

I think AllBalls offers a seal head and RT can set me up with modern valving.
Photo

I don't know where I'll find a piston or seal though. Any ideas?

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10/11/2013 8:39 AM

Can anyone identify this reservoir? It didn't come stock, but was attached to the YZ250 shock.
Photo

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10/12/2013 5:05 PM

Looks great Todd, can't wait to race with you again!
I need an EVO bike.

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10/14/2013 7:51 AM

Hey Mike! I can't wait for next season. I'm working with THOR LAWSON to get my suspension squared away. Wicked nice guy who knows EVO hands down. I'm pretty excited to see how a 1979 bike handles with modern valving etc...

I'll pick away at the rest of the bike over the winter, even though it's pretty close to being track-worthy. I'm even contemplating looking for a spare top end to modify, and a pipe or course. cool

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10/14/2013 9:25 AM

Keep posting , I m looking for an rm, or maybe a yz 250.
Keep an eye out for me up there.
Mike

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10/1/2014 5:27 PM
Edited Date/Time: 10/1/2014 5:28 PM

Nice YZ project - how'd it turn out?

This build is being featured in the BikeBuilds.net directory here: Tdub13's 1979 YZ125 Race Bike

We're currently displaying only a reduced-size photo (similar to what you'd find in Google Images).  We'd like to ask for your permission to publish the original full-size image with your build listing.

If anyone would like to have a build of their own listed, just send me a message with a link to the build thread and we'll take care of the rest.

Ken

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10/2/2014 6:11 AM

Hey Ken,

The bike came out great. I'm very happy with the handling and suspension. The YZ250 forks with RT emulators gave some more travel and are perfect for MX. The rebuilt RT shock with the old school Luft reservoir works great as well.

The motor has good top end, but is lacking on mid and low end somewhat. I have a spare cylinder that I'm modifying doing some heavy work to, to bring it a little closer to modern specs.

Here's what it looked like at the beginning of this season. I raced it at Winchester MX, Wareham MX and even rode an enduro on it. The suspension was definitely NOT set up for the Enduro though. That was a rough one!

I'll post some pics of the cylinder in case anyone is interested. I'm not done yet and don't want to post ugly ports though! So you're going to have to wait a little while for those.

[LINK TO IMAGE]

[LINK TO IMAGE]

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10/2/2014 1:56 PM
Edited Date/Time: 10/2/2014 4:11 PM

Nice bike...

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10/3/2014 1:02 AM

Your bike came out really nice. Good move going the Race Tech route. In 79 I had the RM. Today I'd choose the YZ. The 79 was a one year model. Little lower in seat hieght. Looks like it would turn really well. Was the enduro a vintage enduro or a enduro?

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10/6/2014 9:21 AM

I didn't go directly through Race Tech though because I wanted a little vintage know how and help setting up my LUFT reservoir. I contacted Thor and we figured some stuff out before ordering RT parts. He was a big help.

This picture is all I really had to go on when I found the reservoir. I think this was an LOP bike and there appears to be a White Bro's sticker on the reservoir.
[LINK TO IMAGE]

This is my shock after rebuilding.
[LINK TO IMAGE]

The Enduro's I raced were both Vintage. One was a "short course" which had 1 lap on a sandy MX track and maybe 3 miles of single track in the woods. It was a good hour long race each moto and it was grueling. I got 3rd in that one I think. I also raced a true vintage enduro that I placed well in. There were no trophies in that one so I forget if it was 2nd or 3rd.

Since then I've decided to run this YZ125 on MX only and let my YZ490 take me through the woods.

It's been a great bike so-far and it will be better for next season.

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10/27/2014 5:09 PM

any pics of what you did to the shock ?

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10/28/2014 9:45 AM
Edited Date/Time: 10/28/2014 9:46 AM

The only pics I have are what's above in this thread. I just disassembled it, inspected and cleaned it. Installed RT valving and used RT seal head. I cleaned up the reservoir inside and out. Then assembled. I'm running 175psi of nitrogen.

I also used RT Springs, spring adapter and bumper.

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