1990 RM 125 Rebuild

[Slick_Nick]

One of the many rebuilds I've done in the past few years, I figure I'd share a fun little bike I picked up last spring for a project. The goal of this project was not a full blown restoration on this vintage bike, rather a fun, reliable, "buddy bike" or something I can race in the Evo class once in awhile.

Purchased this clapped out, beaten up, roached, non-running 1990 Suzuki RM 125 for $500. The linkage was removed, the engine was blown, it hadn't run in years. I found it on a local classified site, drove down and made the guy a cash offer, and picked up the bike along with a couple boxes of spare parts.




Like any project, the first step is the teardown. You need to take stock of what you've got, what you need, and what you can throw away. As I progressed, it was apparent there were a LOT of parts on this bike that did not belong. The seat and gas tank were incorrect, almost every single bolt came from a hardware store, and most everything was just finger tight.

[Slick_Nick]

With the engine out and on the bench, we could pull it apart. The previous owner said it had a "fresh top end" (don't they all?) but had never been run. In this rare instance, that turned out to be true. However, I found a janky repair from days gone by of the drain bolt, so decided a full tear down was in order anyways, so I could clean and weld the case half properly.







With the engine apart, and all bearings removed, I could bead blast the aluminum parts, and get to the drain plug repair. I didn't take any pics of the process, but I removed all of the old weld, and lit up the TIG to give it a proper repair, a newly tapped hole, and a brand new drain plug.



The clutch basket cushions were toast, the ring gear was sloppy back and forth. I removed the cushions, modified the basket to use metric screws instead of rivets, installed new cushions, and new screws with loctite and peening to hold them in place.

[Slick_Nick]

With the frame bare, we can turn our attention to getting it stripped. Into the blast cabinet we go, and when it was clean, I discovered a few small cracks. It's a 30 year old chassis, not surprising. out comes the TIG again, got them fixed up, and then we can get to painting the frame. The factory Suzuki welds are ****! The ugly ones in that photo are not mine.

I elected to go with yellow instead of the blue. Even though Suzuki didn't do a yellow frame until the '91 model year, I figured it made for a more updated look. In the 4th photo, you can see the remnants of where someone ground off the fuel tank support bracket. More on this later.






With the frame painted, it sparkles in the sun.

[Slick_Nick]

The gas tank and seat that came on the bike were not for a 1990 RM 125. Based on the part numbers, they were off an RM250. They fit like garbage, so lets go back to stock. Parts for a bike of this vintage are getting quite hard to find, but for about $300 I was able to source the proper fuel tank and seat, and a cheap-o brand universal seat cover off eBay. I suspect this is why the fuel tank support bracket was cut off the frame, I have a better solution later on.



The connecting rod was new, but whomever rebuilt the crank didn't do THAT good a job of truing it. It's "good enough" but hell, let's get it dead nuts on. 125's like to rev after all! Runout before and after.




With the crank trued up, into the deep freeze it went. I cleaned all the parts, inspected the transmission, and installed new bearings where required. Honestly, for a 30 year old machine, it was in pretty good shape. I replaced the crank bearings anyway, despite them looking brand new. I also replated the case hardware, so all the bolts etc were nice and shiny.

[Slick_Nick]

With the bottom end buttoned up, we can turn our attention to the top end. I measured the piston and cylinder with some proper micrometers and a bore gauge, everything was in spec. I still decided to disassemble, clean, and lubricate all the powervalve components anyways. Never trust someone else's work!






Burnt bronze cerakote makes everything look great doesn't it? Engine complete with new gaskets, seals, and hardware where applicable.

[Slick_Nick]

The swingarm bolt that was installed in the bike when I bought it, was not the correct one. It was too small, and someone used (get this!) aluminum sheet as shims! I'm guessing at one point the bolt had to be removed by brute force, or was cut out, and replaced with something that didn't work. I was able to source a new bolt on eBay, as part of a swingarm assembly. The guy didn't want to separate the bolt, so I bought the whole swingarm, had him send me just the bolt, and told him to chuck out the swingarm. He ended up just charging me for the bolt after that.



In addition, the engine mounting bolts were all wrong, sourced some stainless steel socket cap metric bolts locally as well.

I was able to find new swingarm and linkage bearings, so with fresh grease on them, the engine was installed and swingarm slid into place, then once everything found it's home, it was torqued to spec.








Triple clamps installed with cleaned and greased bearings. You can also see the solution I came up with for the fuel tank in this photo, similar to what the KDX uses actually, rubber bumpers on the frame.

[Slick_Nick]

Turning our attention now to the shock, I swear it still had the original oil in it! The stench stung my nostrils when I took it apart!



Bead blasted the body outside, replated the hardware, and painted the spring to match after it was sand blasted clean. Doesn't that new hardware look awesome? The yellow chromate adds some "bling". New seals of course, and shim stack was left stock. Fresh nitrogen charge to top it all off.







With the shock and linkage installed, it's starting to look like a bike! The Suzuki "Full Floater" linkage setup was highly regarded in it's day.

[Slick_Nick]

FRP coming through with the new chain guide.



Turning our attention now to the forks, when I removed them, they had ZERO rebound. I was a little worried that there was internal damage, but it turns out all of the oil had just leaked out. 1990 was the first year for the KYB 43mm's upside down forks on the RM125, they were rebuilt like any other KYB fork. Bushings and seals were trashed, everything was cleaned and replaced with brand new.






The radiators that came with the bike were still the OEM copper/aluminum rads! They were absolutely destroyed. The fins were falling out, they had holes and were smooshed. Unfortunately, OEM radiators are no longer available, and all the used ones are trash, so picked up some GPI (chinesium) ones. I've used these before on the KDX, again, they fit just okay. I installed some NOS Suzuki louvers, and some silicone hoses that needed some trimming to fit properly.

[Slick_Nick]

Got to work installing the coil, CDI box, and wiring harness. Pretty simple on a 125.




Looking now to the fuel system, I cleaned out the new (to me) tank that I bought, rebuilt the petcock, and carburetor. I installed jets for my altitude, and will fine tune later. The carb had sand INSIDE of it.





With the carb installed, I could install the cleaned airbox, and a new filter. This RM125 uses the same air filter as the KDX funny enough, so I had plenty to spare. I also fitted some old (but straight) Renthal's I had laying around. The gold was just coincidence.



Now onto the controls. The levers. These goddamn levers. The front brake was not for this bike. I had ordered a new RM125 lever from Suzuki, and well, it wasn't even close to fitting. The brake line is also supposed to be routed under the fork, much like the KX's of this era. The clutch perch was some chinese thing, but hey, at least it worked. The throttle was not correct either, as I also went to replace the tube and it didn't fit either. Took me several hours worth of fabrication, but I eventually got the throttle to work, and installed the NOS suzuki lever (which I had to modify) into the front master cylinder. Replaced brake fluid and installed fresh pads at both ends. Also installed a new clutch cable, and throttle cable. The yellow grips I lucked out on, finding them in the bargain bin at my local bike shop for 75% off.

[Slick_Nick]

The rims cleaned up nicely, and I replaced a few spokes that were missing and re-trued the wheels. Installed new tubes, and some old take off tires that I had. These would be replaced later. Also had a brand new 520 chain laying around, so installed that. It's a Primary Drive brand. It looks tight as hell in the photo, I am aware. It was also super stiff with the white grease they use in packaging. It loosens up later on, don't worry.





Now, onto the pipe. I looked for a new one I really did, but like any bike of this vintage, pipes are nonexistent these days. The pipe that came with the bike is a "PSI racing" pipe. In order to fit the larger RM250 fuel tank, you guessed it, someone went at this thing with a hammer and dented it in to clear the tank. I sandblasted the pipe inside and out, and welded up the stinger end to install a schrader valve in a washer. I went for strength over looks here. The big end I devised a clamping system. I pumped the pipe up to about 60psi and got after it with the torch. After 15 minutes or so, I got most of the dents out. It's not perfect, but they're about 90% better. I used a band saw to cut off the stinger end. The lower mounting bracket was broken off at some point, so I fabricated a new, stronger one, and welded it on.







A fresh coat of VHT exhaust paint, and we're looking good!

[Slick_Nick]

I fabricated some new pipe mounts using new hardware, and some vibration isolator pads. Fitted new O-Rings in addition to some copper RTV, and fitted the pipe. It's a weird setup how it goes through the frame loop like that.






Was able to get the fuel tank installed as well. The proper tank fit like a glove.

[Slick_Nick]

Now, the silencer. The one that came with the bike was absolutely destroyed. Again, a new one is not available, so I had to get creative here. I decided to make my own. Here is the old one broken down. There was no packing inside. The raw materials for my build are at the top.



Got the canister cut to length, and fit the stinger end piece.



I riveted the cap on with some sealant of course, and built an aluminum bracket and bolted it on, so I could tack the canister into place. I then finish welded them together. My aluminum TIG skills suck.





For the core, I had to get creative. Yes, my metal supplier sells perforated sheet like you'd see in an FMF, etc, but then I'd have to roll it, weld it together, etc. I figured I'd just start with a hollow tube of stainless. Rather than drill a million holes, I used the mill, and cut some slots that are large enough to allow baffling, but not so large that the packing can come through.

[Slick_Nick]

With fresh packing installed and the end cap fitted, the silencer was finished. Who needs FMF when you've got SNF. (Slick Nick Fabrications)






New hardware all around. I decided to do the entire bike with metric socket cap style hardware.

[Slick_Nick]

The absolute most fun part of any build is the installation of the plastics. Its when the bike takes it's "final shape" and I often leave this part until the very last. I was able to source a brand new full plastics kit from UFO Italy. Ordered directly through them. Shipping to Canada actually wasn't too bad! I had to wait 6 weeks for them to arrive, but when they finally did I was blown away. This is actually a pretty good looking bike! I added the white vinyl number plates. I decided against doing some EVO-MX decals, as I wasn't sure what direction I wanted to take the bike just yet.

[Slick_Nick]

Got the fork guards sorted out, and I present to you, the finished bike!

[Slick_Nick]

[Slick_Nick]

The first start was uneventful. Bike ran like a top, although super rich from what I could tell going up and down the street. Ended up 3 steps leaner than stock on the pilot, 2 steps leaner than stock on the main. Ran amazing after that. Loaded 'er up and took it to the track for a proper break in.



Unfortunately, after beating the piss out of it for a good 20 minute moto, it seized on me in the air. I pulled the clutch in just in time and was able to keep from crashing, despite the nosedive. I looked down, and my left boot was covered in coolant. The radiators were ice cold. Upon closer inspection, one of the head studs was missing. Good thing I had another bike that day! Got it home, cleaned it up, and went to diagnose the issue.






I got lucky. Once I had the bike clean, I tried kicking it over, and it actually fired up on the first kick. I immediately shut it off, and decided to do a top end anyways. The piston was toast, the cylinder was in great shape. I deglazed it, ordered a new piston and gasket set. Remember before, when I said "don't trust other people's work?" Yeah, well I had assumed that the head studs were installed properly. I never checked them. In this engine, the studs go down into the water jacket, so when the one stripped out, it let all the coolant spray out, allowing the bike to overheat. I was in the middle of a good moto, so I never thought to look down, I had already done a few shakedown motos, with no issue.

At this point, I removed all the studs, repaired the stripped hole, checked the rest, and installed new studs where reuired, using a thread sealing compound. Installed a new top end a week later, and she was back in business.

[Molly's 70]

This is absolutely Fantastic!!! Good stuff here.

[Slick_Nick]

I rode the bike on and off again most of the summer, although it was limited to just a few laps here and there for fun, it mostly just sat in my garage looking nice, but on the odd occasion I'd bring it out, get my 125 fix for a moto or so, then switch back to the KXF. I swap tires frequently, so when the KXF got new shoes I'd put the takeoffs on the RM.




Had a local photographer approach me one day, he had taken some nice shots of me on the RM. The pipe is really weird. Your boot sits directly on it.

[Slick_Nick]

After the bike had probably, oh I dunno, 5 or 6 hours on it, another issue happened. I was riding it pretty hard, and noticed a sudden "rattle" from inside the engine. I shut it off immediately, and went home to clean the bike and see what the issue was. Once I got the head off, I could see that one of the power valves had broken. I later learned that this was a VERY common issue on these bikes, I'm just lucky it didn't drop into the cylinder! The valve snapped where they all snap, and it was hitting the piston, making that rattle.






I had some work to do. This powervalve is unique to the 1990 model year, and these parts simply aren't available anymore. Because them breaking was a common issue, there were no used ones either. They were all broken. So, I got to cleaning, and fabricating. The hole for the pin is also at an offset angle relative to the valve, not to mention holding a part like this in the lathe took ages to get it zero'd out in the chuck. After a whole day, I had cut down the valve, built it up with 4043, turned it down, and drilled the pin hole. The face of the valve was slightly rounded due to hitting the piston, so I took the edge back as best I could.





At this point, I also realized that the plastic spring collar for the valve was missing, and its no longer available from Suzuki. So, I made one from aluminum. All parts were cleaned, and the damage removed as best as I could.