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My brother and father purchased a pair of YZ125s in '79 so they could go dirt bike riding together. After Dad got bruised up pretty good once and took him several days to heal (that's a very funny story in itself) he sold the YZ for self preservation reasons and my best friend and brother-in-law Kent Friedrichsen bought it from Dad. Kent and Mike had way to much fun on those bikes for a lot of years. They were riding in the creek one day when Kent's YZ died. Turns out a rock punctured the side cover and damaged one of the charge coils for the ignition.
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When the "4 Bandelaros" started racing the YSRs and it got to be a little boring because we had gotten to the point where everybody could hold the throttle open for just about every part of the track, we all kind of decided it was time for a change. I remembered Kent's bike and it had been sitting now for probably 3 years at this point. Since my family and I were going on vacation the following week, I bought the bike and transplanted the engine.
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I tried all kinds of different motor mounts to help alleviate some of the vibration. On one of my other projects I had used a rubber/plastic material to dampen out some of the vibration and it worked pretty good. I tried it on the top rear mounts and had some fairly satisfactory results. They never broke, helped with the vibration and didn't allow the engine to move around, they did look bad though. In fact, I caught a lot of grief over that.
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It went together pretty easily actually, there really wasn't much to it. The biggest thing was finding a spot for the pipe. I had originally run it under the bike but quickly ground a hole thru it. I then removed the right side brace of the chassis around the steering head and ran the pipe thru the frame out the back. It didn't seem to change the handling very much and sure made for better ground clearance.
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The next thing was a brake. It was very obvious that the engine was way above what the brakes could handle and I HAD to do something. I started rummaging around the office looking for a big brake rotor I could use. There were several that would have done the job but only 2 that would have WORKED, if you know what I mean. I settled on a FZR400 front brake rotor, the other choice was a FZR1000 rear rotor. The 400's rotor had a smaller inner diameter that would fit nicer.
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I had a friend make an aluminum carrier to bolt to the stock YSR hub that would allow the FZR rotor to bolt to it. I got an opposed 4 piston differential bore brake caliper from a FZR1000 and the master cylinder off the same year bike. I made a hanger for the caliper and had to machine the caliper to clear the aluminum rim, it cleared the steel wheel, and made a steel braided brake line.
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The rim diameter of a YSR rim is 256 mm, the rotor is 240 mm you have to take the rotor loose from the wheel and the caliper loose from the carrier in order to get the front wheel off but, it's all worth the effort the first time you squeeze the front brake lever. The braking performance outshone the engine performance 2-1. It stopped with unparalleled power and ability with a feel that is beyond reproach. Even the OW isn't in the same ball park let alone the same league! It doesn't matter what tires you use, you can tell EXACTLY when the wheel is about to lock.
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To give you an idea of the performance of this bike, Al and I took our YSRs to the drag strip, Al's 80 and my 125. The 125 went 13.85 at 102 mph after I nearly stalled it at the start. I actually had to pedal with my feet in order the catch the motor, I happen to be racing a 5.0L Mustang GT. He, of course, got the hole shot, the YSR caught him about the 1/8 mile mark and went right on around. I remember looking over at the car (level with the door handle!) as I was passing him and the passenger (!) looked over at me and then turned to the driver and started yelling at him. It was hilarious! The really bad part of this is that the YZ80 water cooled engine from any year is actually faster. It doesn't have the grunt coming off the corners but it is faster on top.
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I took the bike to Blackhawk Farms once too and ran a whole string of 1 min, 23 sec laps. I was running with a bunch of FZR400s for the first 8 laps. They would pass me on the front straight only to start braking on the #6 marker for turn 1, I would go all the way down to inside the #1 marker, pass them all and then leave them until the front straight again. I had more fun in those 12 laps than I can ever remember having on a bike. I eventually got to the point towards the end of the 8th lap where they couldn't catch me anymore on the front straight. They were running 1.24s and 25s.
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The rear used a stock shock for it's spring and I used a Fox Air Shock (remember those?) for the rear to control the damping, it worked very well although it was quite heavy. It had TCR top clamp and brace. IRC slicks too.
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I sold the bike to Paul Gardalen in 94 or 95 maybe, and he has run it all these years and finally retired it this fall. Paul changed the pipe and the rear shock and went to an aluminum 1" longer swingarm. He also went back to the steel front wheel because the aluminum wheel spreads the tire too far and slows the steering.
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Paul's bike is the "zero" bike following and leading my 426 in some of the photos on the action page of the 426. He is also the rider of the 426 in yellow on the action page while I took the pictures for the magazine article. Paul sold the YZ125 motor and is now using a '87 YZ250 engine out of a flat tracker.
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